ࡱ>  zy   !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~Root Entry F WordDocument  CompObj^ld Proxima Centauri be from the Sun? Give your answer in kilometers using powers-of-ten notation. 22. How many Suns would it take laid side by side to reach the nearest star? Use powers-of-ten notation. See the preceding question. 23. The average distance from the Earth to the Sun is 1.496 x 10 8 km. Express this distance (a) in light-years and (b) parsecs. Use powers-of-ten notation. (c) Are light years or parsecs useful units for describing distances of this size? Explain. 24. The speed of light is 3.00 X 10 8 m/s. How long does it take light to travel from the Sun to Earth? Give your answer in seconds using powers-of-ten notation. 25. When Voyager 2 sent back pictures of Neptune during its historic flyby of that planet in 1989 the spacecraft's radio signals traveled for 4 hours at the speed of light to reach Earth. How far away was the spacecraft? Give your answer in kilometers using powers-of-ten notation. See the preceding question. 26. The star Epsilon Eridani is 3.22 pc from Earth. (a) What is the distance to Epsilon Eridani in kilometers? Use powers-of-ten notation. (b) How long does it take for light emanating from Epsilon Eridani to reach Earth? Give your answer in years. You do not need to know the value of the speed of light. 27. The age of the universe is about 15 billion years. What is this age in seconds? Use powers-of-ten notation. *28. Explain where the number 206,265 in the small angle formula comes from. *29. At what distance would a person have to hold a nickel which has a diameter of about 2.0 cm in order for the nickel to subtend a angle of (a) 1 ? (b) 1 arcmin? (c) 1 arcsec? Give your answers in meters. *30. The average distance to the Moon is 384,000 km and the Moon subtends a angle of 1/2 ? Use this information to calculate the diameter of the Moon in kilometers. *31. Suppose your telescope can give you a clear view of objects and features that subtend angles of at least 2 arcsec. What is the diameter of the smallest crater you can see on the Moon? See the preceding question. *32. A person with good vision can see details that subtend a angle of as small as 1 arcminute. If two dark lines on a eye chart are 2 millimeters apart how far can such a person be from the chart and still be able to tell that there are two distinct lines? Give your answer in meters. *33. On December 11, 2000 the planet Venus was at a distance of 0.951 AU from Earth. The diameter of Venus is 12,104 km. What was the angular size of Venus as seen from Earth on December 11, 2000? Give your answer in arcminutes. Discussion Questions 34. Scientists assume that reality is rational. Discuss what this means and the thinking behind it. 35. How do astronomical observations differ from those of other sciences? WEB/CD-ROM Questions 36. Use the links given in the Universe web site Chapter 1 to learn about the Orion Nebula Figure 1-5. Can the nebula be seen with the naked eye? Does the nebula stand alone or is it part of a larger cloud of interstellar material? What has been learned by examining the Orion Nebula with telescopes sensitive to infrared light? 37. Use the links given in the Universe web site Chapter 1 to learn more about the Crab Nebula Figure 1-6. When did observers on Earth see the supernova that created this nebula? Does the nebula emit any radiation other than visible light? What kind of object is at the center of the nebula? 38. Use the links given in the Universe web site Chapter 1 to learn more about TOPEX/Poseidon see Figure 1-13 and other space missions that study the Earth. What are a El Nino and a La Nina? How can they be detected from space? How can the speed of winds over the oceans be measured by a orbiting satellite? 39. Access the AIMM Active Integrated Media Module called Small Angle Toolbox in Chapter 1 of the Universe CD-ROM or web site. Use this to determine the diameters in kilometers of the Sun, Saturn, and Pluto given the following distances and angular sizes. Object Distance km Angular size (") Sun 1.5 X 10 8 1800 Saturn 1.5 X 10 9 ܥe# d 0, l, l   (   T Times New Roman Symbol ArialCourier NewTimes New Roman ArialMeta-NormalMeta-BoldTimes New RomanMeta-Caps SymbolHET 602 Exploring the Solar System The textbook for HET602 is either of the following books: Universe, Kaufmann & Freedman, 6th edition, 2001, (New York: W.H. Freeman & Co.). ISBN 0-7167-4647-6. The earlier 5th edition is also suitable. The New Solar System, Beatty, J. K., Petersen, C. C. & Chaikin, A. (eds), 1999, 4th edition. Chapter 1 Review Questions 1. Describe the role that skepticism plays in science. 2. What is the difference between a theory and a law of physics? 3. What caused the craters on the Moon? 4. What role do nebulae like the Orion Nebula play in the life stories of stars? 5. What is the difference between a solar system and a galaxy? 6. What are degrees, arcminutes, and arcseconds used for? What are the relationships among these units of measure? 7. How many arcseconds equal 1 ? 8. With the aid of a diagram, explain what it means to say that the Moon subtends an angle of 1/2 ? 9. What is an exponent? How are exponents used in powers-of-ten notation? 10. What are the advantages of using powers-of-ten notation? 11. Write the following numbers using powers-of-ten notation: (a) one hundred million (b) forty thousand (c) three one-thousandths (d) twenty two billion (e) your age in years. 12. How is an astronomical unit (AU) defined? Give an example of a situation in which this unit of measure would be convenient to use. 13. What is the advantage to the astronomer of using the light-year as a unit of distance? 14. What is a parsec? How is it related to a kiloparsec and a megaparsec? 15. Give the word or phrase that corresponds to the following standard abbreviations (a) km (b) cm (c) s, (d) km/s (e) mi/h (f) m (g) m/s (h) h (i) y (j) g (k) kg. Which of these are units of speed? Hint: You may have to refer to a dictionary. All of these abbreviations should be part of your working vocabularly. 16. In the original 1977 Star Wars movie Han Solo praises the speed of his spaceship by saying It's the ship that made the Kessel run in less than 12 parsecs. Explain why this statement is obvious misinformation. 17. A reporter once described a light year as the time it takes light to reach us traveling at the speed of light. How would you correct this statement? ADVANCED QUESTIONS The small angle formula given in Box 1-1 relates the size of a astronomical object to the angle it subtends. Box 1-3 illustrates how to convert from one unit of measure to another. A object traveling at speed v for a time t covers a distance d given by d=vt for example a car traveling at 90 km/h (v) for 3 hours t covers a distance d = 90 km/h (3h)= 270 km. Similarly the time t required to cover a given distance d at speed v is t = d/v for example if d = 270 km and v = 90 km/h then t = 270 km/ 90 km/h = 3 hours. 18. What is the meaning of the letters R I V U X G that appear under some of the figures in this chapter? Why in each case is one of the letters highlighted? Hint See the section How To Use This Textbook that precedes Chapter 1. 19. A hydrogen atom has a radius of about 5 X 10 -9 cm. The radius of the observable universe is about 15 billion light years. How many times larger than a hydrogen atom is the observable universe? Use powers-of-ten notation. 20. The Sun's mass is 1.99 X 10 30 kg three quarters of which is hydrogen. The mass of a hydrogen atom is 1.67 X 10 -27 kg. How many hydrogen atoms does the Sun contain? Use powers-of-ten notation. 21. The diameter of the Sun is 1.4 X 10 11 cm and the distance to the nearest star Proxima Centauri is 4.2 ly. Suppose you want to build a exact scale model of the Sun and Proxima Centauri and you are using a basketball 30 cm in diameter to represent the Sun. In your scale model how far away wou 16.5 Pluto 6.3 X 10 9 0.06 Observing Projects 40. On a dark clear moonless night can you see the Milky Way from where you live? If so briefly describe its appearance. If not what seems to be interfering with your ability to see the Milky Way? 41. Look up at the sky on a clear cloud free night. Is the Moon in the sky? If so does it interfere with your ability to see the fainter stars? Why do you suppose astronomers prefer to schedule their observations on nights when the Moon is not in the sky? The reflecting sunlight coming from the moonlight can cause interference. 42. Look up at the sky on a clear cloud free night and note the positions of a few prominent stars relative to such reference markers as rooftops, telephone poles, and treetops. Also note the location from where you make your observations. A few hours later return to that location and again note the positions of the same bright stars that you observed earlier. How have their positions changed? From these changes can you deduce the general direction in which the stars appear to be moving? 43. Use the CD ROM that accompanies this book to install the Starry Night planetarium software on your computer. Use Starry Night to determine when the Moon is visible today during the day and when it is visible tonight. Determine which if any of the following planets are visible tonight Mercury, Venus, Mars, Jupiter, and Saturn. (1) Feel free to experiment with Starry Night. You can always return to your starting screen by clicking on the Home button in the Control Panel at the top of the main window. To see the sky from your actual location on Earth select Set Home Location in the Go menu and click on the Lookup button to find your city or town. (2) To change your viewing direction move the mouse until the cursor changes into a little hand. Then hold down the mouse button on a Windows computer the left button as you move the mouse and you will move the sky. (3) Use the Control Panel at the top of the main window to change the time and date that is displayed as well as how rapidly time appears to change. (4) Use the Find command in the Edit menu to locate specific planets or stars by name. (5) To get more information about any object in the sky point the cursor at the object and double click the mouse on a windows computer click the right button. The Starry Night Backyard Manual, which can be found in the Starry Night Backyard program on the CD, contains detailed directions on how install the Starry Night Backyard software and how to use its various features. This users guide is accessible from the Help item on the menu. It might be useful, and certainly good scientific practice, to keep a notebook specifically for these exercises in which you can write down the date and time of each observation as well as notes on set-up procedures as you become familiar with them. Starry Night Backyard Project: Getting Acquainted Open Starry Night Backyard. Your screen will now display what is called a Starry Night Backyard document. This consists of four sections: 1.The title bar at the top shows the program name and version as well as controls appropriate to your computer operating system for closing or resizing the window on your monitor screen. 2.The menu contains all of the menu commands available in Starry Night Backyard. 3.The control panel displays, and allows you to change, many of the important parameters of the document. 4.The main view window comprises the majority of the document window and shows a virtual view of the sky consistent with the parameters shown in the control panel. The first time you open the program, you will be prompted to set your home location. On the Menu line of the display, click on Go/Set Home Location. Click on Lookup and search the displayed list for the location closest to your home. For example, if you live in New York City, click on New York, USA, and click OK. Note that a red circle shows this location on the world map. Click Set Home Location to place yourself at home on Starry Night Backyard. [If you cannot find a suitable location close to your home, close this list by clicking on Cancel. Type in the name of your location and enter your latitude and longitude, obtained from an atlas or an Earth globe, in the appropriate boxes. It order to define the time correctly, you will also need to enter a value in the Time Zone box. Time zones are calculated according to the time difference with Greenwich in London, England. Since a time zone is 15 wide on Earth, the number of hours to enter in the time zone box will be your longitude divided by 15 to the nearest integer, being negative if west of Greenwich. Thus, Eastern Standard Time is 5 hours behind Greenwich, so you would enter "-5h" if you were on Eastern Standard Time. Click on Add to record your location in the list, and click Set Home Location to go to this location. Now, Starry Night Backyard will always open at this location unless you change this setting. These time zones are discussed in Section 2-7 of Freedman and Kaufmann, Universe, 6th Edition. The North American time zones are shown in Figure 2-21 when it is midday in England. Further advice on this set-up is given in the Starry Night Backyard Manual on the CD.] The program will open with a display of the sky at your location for the time and date set up on your computer system clock and with time running at a normal rate. This time and date are displayed on the control panel. Also shown are time controls similar to those on a VCR. These allow single time-step backward and forward, continuous running backward and forward, stop and real-time running forward. The time-step for backward and forward motion can be chosen from a list displayed when you click on the time-step box. You should experiment with these controls. In particular, at this time, you should stop time advancing by clicking on the Time-Stop button. (Note: The rate of continuous backward and forward motion will depend to some extent upon your computer operating system. This rate and other similar parameters can be adjusted by going to File/Preferences/Scrolling/Time.) Field of View controls to the right of the control panel permit zooming in and out or setting the field of view from a scroll-down list, displayed when you click on the Field of View box. Click on Window/Planet List on the control panel to display the list of planets and planetary system objects. If it is daylight at your location when you do this project, turn the daylight off by clicking on Sky/Daylight on the control panel. [Hint: If you want to return to a particular set-up, you can save it as a Starry Night Backyard File with a name *.snf in an appropriate location on your computer, substituting any suitable name for the *. You can then recover this set-up at any time by using File/Open.] Click on Labels/Planets to display labels next to the planets and the Sun. [Hint: You might want to change the labels for better contrast by clicking on Labels/Label Settings/Planets and changing their color and/or type. Click on OK to save the changes.] 1.What planets, if any, appear in your southern sky (the default view) from your home location? [In the southern hemisphere, this will be your northern sky!] If you are using a notebook, note date and time and the names of the visible planets. 2.What is the brightest star (other than the Sun) that appears in the southern sky from your home location? (Again, enter this name in your notebook.) To answer this second question, you can use a powerful function built into the Hand Tool- the hand-shaped icon that shows the position of the mouse on the screen. Whenever this icon is moved close to an object, it will change to an arrow and automatically display the objects name and other details. In the case of stars, the appropriate constellation name and distance (if known) will appear. You should experiment with this function. Find the Andromeda galaxy by clicking on Edit/Find in the menu box and typing "andromeda galaxy" in the Name contains box. Starry Night Backyard will automatically zoom in on this nearby galaxy, but you can also adjust the field of view using the zoom controls in the Control Panel. (If this object is below your horizon, Starry Night Backyard will ask whether you want time adjusted to bring this object into view. If this occurs, click on Yes.) 3.There is a bright circular object at the lower left of Andromeda. What is this object? You should experiment with the various controls and choices provided by Starry Night Backyard. In particular, you should use the Hand Tool to move your direction of view. Clicking on the mouse changes the hand icon to a closed fist. Movement of this mouse will now result in the sky moving to provide you with a different Gaze direction, the coordinates of which are displayed at the bottom of the screen as this movement is proceeding. Take some time to just "play" with the program. Try different views of the sky. Locate an object in which you're interested (e.g., a planet) and zoom in on it to see its surface and/or moons. Change the viewing date, time, or location by clicking on the relevant boxes and resetting values. Change the Time Step to 1 Day and watch the movement of the planets across the sky. (These are just suggestions; use your imagination!) Take notes on your observations; your instructor may ask you to share them with the class. This Web site includes Starry Night Backyard Projects for most of the textbook chapters. Alternate notes on The Motion of the Planets and Gravitation This is a good set of notes on motion and gravitation, but the references in it refer to a text book we are not using this term, Universe, by Roger Freedman and William Kaufmann, 6th edition. I'm keeping it up, though, because you may find parts of it useful. Note that if you go back to the schedule page you find a different set of notes with references to this term's book, The Solar System: The Cosmic Perspective, by Bennett et al. This topic is a tough one, but it is intellectually very exciting because it introduces Isaac Newton's laws of motion and gravity. So far, the book has just given us descriptions of what we see in the sky, but in this chapter, we get causal explanations for what we see. After studying this, you'll have a mental map of the location and motion of the planets in solar system, and you'll also know why they move this way. You'll even have a better understanding of how objects move (and why they fall down) in our ordinary life on the surface of this earth. This chapter has a couple of mathematical equations, but the math is just a means to an end. The important thing is learning what the math means. The most important parts of Chapter 4 are: Sections 4-2, 4-3, and 4-5 through 4-8. You may omit Boxes 4-1, and 4-2. This chapter has a particularly good set of animations posted at the publisher's web site , or on the CD-ROM that came with your text book. The link just above will take you to the right chapter of the web site, but you will still have to click on "Animations and Videos" near the bottom of the column at the left of the window. Figures 4-5 and 4-9 are ones I might ask you to redraw and explain on an exam. The Apparent Motion of the Planets: In Section 4-1, it is important to know what the planets appear to do. To an observer on the Earth, the planets Rise in the east and set in the west daily. This looks a lot like the motion of the Sun, Moon and stars, and like their motion, it is really due to the Earth's spin. This is an obvious and rapid motion, that you can see in just a few hours watching the night sky. The planets also move slowly compared to the stars, as do the Sun and Moon. If, for example, Mars appears at one time to be traveling with the stars of Pisces, some months later, you will find it traveling with the stars of Taurus. This is a slow change; you will have to watch for days or weeks to detect the change. The motion of the planets against the stars can be from west to east, which we call direct, because it is the same way as the Sun and Moon move. from east to west, which we call retrograde. The planets alternate between direct and retrograde motion in a way that looks at first to be pretty irregular. An example is Figure 4-2, which shows how Mars will move between July of 2005 and March of 2006. From October 1 to about December 1, we say that "Mars is in retrograde motion." An important thing to understand about this figure is that Mars doesn't follow this path between July and March of every year, or even at any other exactly regular interval. Mars will spend a few months in retrograde motion roughly every couple of years, but it will happen in different parts of the sky. Animation 4-2 gives another such example, and it's in motion! Sometimes instead of tracing a zig-zag in the sky Mars will trace a loop, and the loop or zig-zag can be tall or short, fat or thin--lots of different shapes. All the other planets also exhibit retrograde motion, too, and they also do it in a sort-of-but-not-very regular way. The reason I emphasize this is that predicting the motion of the planets was one of the key problems of ancient science. One of the solutions was the Ptolemaic (geocentric, or earth-centered) system described in your book. Do check out animations 4-1 and 4-2 to get a quick idea of how this system worked. If this were a course in the history of science, it would be fascinating to consider the Ptolemaic systems in greater detail; for more than 2000 years it was the crowning achievement of western and Islamic science. But since we no longer believe it, it isn't really part of this course. The True Motion of the Planets is Heliocentric: The single most important idea to take away from section 4-2 is that even though the planets, Sun and Moon may seem to orbit (move around) the Earth, in reality, the planets (including the Earth, and its satellite, the Moon) orbit the Sun. This explanation is also more than 2000 years old, but it was not widely believed until about 400 years ago. The heliocentric system is often called the Copernican system, after Nicolaus Copernicus. He worked out a detailed version that was appealing because it sorted out the order of the six planets then known from Mercury (orbiting closest to the sun) through Venus, Earth, Mars, and Jupiter, to Saturn (farthest). This also turns out to be the order of the speed of their motion. (Mercury is fastest and Saturn is slowest.) In addition, it is the order of their orbital periods. (Mercury takes only 88 days to orbit the sun, while Saturn takes almost 30 years.) The planets that have been discovered since Copernicus's time follow the trend; check out their sidereal periods in Table 4-1. Section 4-2 has a great deal of vocabulary about conjunctions, elongations and oppositions. Don't memorize it. What I want you to know is the basic geometry: The planets with orbits smaller than the Earth's must seem to us to be roughly in the same direction as the Sun. Mercury, in fact, orbits so close to the sun that most of the time it is pretty hard to see because of being lost in the Sun's dazzle. Venus is easily identified as the "evening star" or "morning star," because it can alternately be seen a little above the western horizon soon after sunset or above the eastern horizon soon before sunrise. They can never be seen opposite to the sun, which means they can never be seen high in the sky at midnight. (To test your understanding, use Figure 4-6 to explain to yourself or a friend when and why these planets are easiest to see.) The planets with orbits larger than the Earth's can be in the same direction as the Sun, opposite to it, or anywhere in between. (Test yourself: which planets are these? Using Figure 4-6, pick out different points at which these planets might be, and decide at what time of day and in what direction you might look to see them.) The other important thing to understand is that the heliocentric system also has good explanation of that old puzzle, the retrograde motion of the planets. Check out Figure 4-5, which shows the faster -moving Earth passing the slower Mars. The numbered pairs of points indicate where the two planets are at successive times, and the yellow lines are lines of sight from the Earth to Mars. The yellow lines are extended out to show where Mars would appear against the background of the distant stars, and they show that as the Earth passes Mars, Mars appears briefly to reverse its motion against the background of the stars. This is the true explanation of retrograde motion as described in Section 4-1 and Figure 4-2. After you've gotten the general idea of Figure 4-5, check out animation 4-4 on the Universe CD-ROM that came with your textbook, or at the publisher's web site . Figure 4-5 is one I might ask you to redraw and explain on an exam. You don't need to worry much about synodic periods, because the planets' sidereal periods will be the only ones we will use in the rest of the course, and you may omit Box 4-1. Support for the Heliocentric System: The idea that the Sun, not the Earth, was at the center of the universe was a pretty radical one. It took persuasive new observations and new theory to to persuade scientists that it was right. (Remember what we read in the first week: for a theory to be scientific, it must testable. To decide between two theories, you need to decide which one is better supported by observation.) The two most important astronomical observers of the time were Galileo Galilei and Tycho Brahe. Galileo was (as far as we know) the first person to use a telescope to look at the sky. He made a tremendous number of discoveries, but the two that offered the best support for the Heliocentric system were his observations of the phases of Venus and of the satellites of Jupiter. Take a careful look at figures 4-8, 4-9, and 4-10. By now you should know enough about phases that you can sort out why the fact that you can sometimes see Venus in its nearly full phase demonstrates that Venus sometimes gets on the far side of the Sun from the Earth. This supports the idea that Venus really orbits the sun. (If this isn't clear to you, you might want to review the phases of the Moon, but keep in mind that the two situations are different: the Moon orbits the Earth, and its full phase comes when the Moon is on the opposite side of the Earth from the Sun, while the nearly full phase of Venus appears when Venus is in about the same direction as the sun.) Figure 4-9 is one I might ask you to redraw and explain on an exam. The moons of Jupiter look like a heliocentric system in miniature. The biggest body (Jupiter) is in the middle; the fastest satellites obit closest to it, and the slowest farther out. By offering a mini model, Jupiter's system was thought to help confirm the Heliocentric System. The main thing to understand about Tycho is that even though he didn't use a telescope, his observations of the planets' positions were the most accurate that had ever been made. Kepler took them at Tycho's death, and used them to develop his three laws of planetary motion. Other that this idea, you needn't worry much about Section 4-4. Kepler's Laws of Planetary Motion: Johannes Kepler was a mathematician and a theorist, not an observer, but when Tycho died, Kepler used Tycho's data to develop three laws that make it possible to calculate the planets' motions very exactly. Each planet orbits the Sun in an ellipse, with the Sun at one focus of the ellipse. This was new and different, because before Kepler, people usually assumed that orbits were circles. To understand the difference, look at Figure 4-16. Get yourself a pencil, paper, a loop of string, and two thumbtacks or push pins, and draw some ellipses for yourself. Play with changing the distances between the tacks, and the length of the loop of string. I want you to understand that a circle is a special kind of ellipse. In Part (b) there are four blue ellipses with different eccentricities. Notice that an ellipse with e = 0 is a circle, and as e approaches 1, the ellipse is stretched out almost to a line. Figure out how to use the string, tacks and pencil to draw an ellipse that is the same as a circle. You need to understand two other things: The orbits of the planets truly are ellipses, but they are ellipses that are pretty close to circles. (That's probably why in thousands of years of thinking about the orbits, nobody before Kepler had caught the difference.) In this class, we will often approximate the orbits of the planets as circles. The sun is not in the center of the ellipse; it is at one focus. Figure 4-17 shows this in an exaggerated way. No planet has an orbit that is so eccentric an ellipse, though an asteroid could easily have that eccentric an orbit. See how far off center the Sun is? Of course, when we approximate the planets' orbits with circles, we treat the Sun as though it were at the center. (Test yourself : What's at the other focus? (((There's nothing at the other focus. It's empty!) A line joining a planet and the Sun sweeps out equal areas in equal times. To understand this, look at Figure 4-17, and animation 4-6 on the Universe CD-ROM that came with your textbook, or at the publisher's web site . The math that goes with this is to messy for us, so as far as I'm concerned what you need to remember is this: A planet travels fastest when it is closest to the Sun (at perihelion), and slowest when it is farthest from the Sun (at aphelion.) Replay the animation and see that this is what is shows. P2= a3. OK, this isn't a math class, and we will hardly use this equation at all. You can omit Box 4-2. Instead, remember these ideas: This is a relationship between the sizes of orbits and their periods (the amount of time it takes to go once around.) That means that for any given size of planetary orbit, there is one possible period, and for any given period, there is one possible size. If that combination isn't what the planet has, it isn't in an orbit. (Other possibilities include falling into the Sun or escaping the solar system altogether.) Planets orbiting close to the sun travel quickly and have short periods. Planets orbiting far from the Sun travel slowly and have long periods. (Remember this idea? Copernicus sorted the planets' order out based on orbital period, but he didn't actually have an equation relating size of orbit and orbital period.) Here's a more philosophical idea. Because there is one numerical equation that we can use to to relate size and period for all the planets that orbit the sun, we know that there must be something in common that ties all those orbits together. As we will see when we get to Newton's laws of motion, that common something is the mass and gravitational attraction of the sun. It will turn out that it is the gravitational pull of the Sun on each of the planets that pulls the planets into solar orbit. The words period, aphelion, perihelion and eccentricity are ones you need to know. Newton's Laws of Motion: Newton's laws are different from Kepler's laws. While Kepler's laws describe motions in the very specific situation of the planets orbiting the Sun, Newton's laws describe the motion of bodies in just about all the situations we might consider "normal." you, sitting in your chair your car, driving down the road the puck, in a game of hockey a spacecraft, exploring the solar system the planets, orbiting the sun. everything else that's neither so tiny, nor so extremely dense, nor so extremely fast-moving that we need quantum physics or Einstein's general relativity to describe it--and in this course we needn't worry much about things like that. Newton's laws address the behavior of matter in ways so fundamental that we think of them not just as describing motion, but as actually governing it. They are important to understand. You should read Section 4-6 and Box 4-3 carefully, really thinking about all the examples and the vocabulary. First law or Law of Inertia: A body remains at rest, or moves in a straight line at a constant speed, unless acted on by a net outside force. Usually on Earth things do slow down due to an outside force: usually friction or air resistance. If you remove friction, things keep moving. In space, where there is effectively no friction, craft can travel long distances without being pushed all the time. Usually, a rocket fires to start the spacecraft on its way, then the spacecraft coasts until it reaches its destination. Whenever the speed or direction of a body's motion does change, we should be able to find a force that caused the change. The planets don't move in straight lines at constant speeds. They orbit in ellipses, changing both speed and direction from moment to moment, so we know a force must be acting on them. Second Law: F = ma. This tells us how much change of motion to expect from a certain force, as long as we know the mass of the moving body. First, we need some vocabulary. Check out the definitions of force, speed, acceleration and velocity on page 78, and the definition of mass on p. 80. [They're very important, so I'll put some of them here. Force is the amount of push a body receives. Mass is the amount of material in a body. Acceleration is the rate at which a body's velocity (speed and direction) changes.] Note that mass is different from weight. We can rewrite the second law in words: It takes a big force to make a big change in motion. It takes a big force to change the motion of a very massive body. Note that a body accelerates any time you change either the speed or the direction. This includes speeding up, slowing down, or turning a corner. Constant circular motion involves acceleration, because the direction is always changing. The units of acceleration are velocity per unit time, or m/s/s = m/s2. From F = ma, the units of force must be mass x acceleration = kg x m/s2 = Newtons (N). Third Law or Law of Action and Reaction: Whenever one body exerts a force on a second body, the second body always exerts an equal and opposite force on the first body. This law is simple but subtle. Whenever a body exerts a force on another, there is a "reaction force" acting on the original body. When you push on an object, it pushes back on you. That's why it hurts when you fall down. When you hit the ground, the ground hits you just as hard. A rocket works on this principle as well. The force pushing the gas out one side is balanced by a reaction force pushing the rocket forward. Note that there doesn't have to be anything for the rocket to push against. The "push" comes from the expanding gas in the rocket, not the air, ground, or anything else that might be behind it. So far, so good. Here's the subtle part: Consider the problem of planetary orbits. Orbits are curved, so the Sun must be exerting a force on the planets. But the planets must also be exerting an equal force on the Sun, causing it to move. Since the planets are much smaller than the Sun, the Sun's motion is much smaller than the planets. At first it may be hard to believe that the force that the whole Earth exerts on little you is just the same size as the force that you exert on the Earth. But it's true. It's equally true that the force that the Earth exerts on the absolutely enormous Sun is just the same size as the force the Sun exerts on the Earth. Think up your own illustrations for Newton's laws. Read Box 4-6, then think about something you often do (throwing a ball, driving your car, etc...) Come up with illustrations of each law, and be as specific as you can in thinking about these questions: what body exerts the force? What body experiences the force? Is it a big force or a little force? Is it exerted on a high or low mass body? For example, imagine a rowboat and a freighter, both traveling at the same speed. Imagine what happens when the engine of the freighter is stopped, and the rower stops rowing. For a moment, both boats coast (inertia). But even though there's no motor, there is a force acting on both boats--friction of the water-- and both boats slow down. The water's friction brings the low mass rowboat to a complete stop in moments. However, it takes a much bigger force to bring the very massive freighter to a stop, and it will coast for much longer (2nd law). In fact, to stop a freighter quickly, the captain often calls for a tugboat (a little boat with a huge motor) to come and push backwards on the freighter. Use the third law yourself to decide how much the freighter pushes on the tugboat. Let's do a numerical example: A jet moving 1 km/s comes stops in 20 seconds. If the jet's mass is 10,000 kg, what force is exerted on it? The jet's speed is 1000 m/s. The acceleration = change in speed/unit time = (1000 m/s - 0 m/s)/20 seconds = 50 m/s2. The force F = m x a = 10,000 kg x 50 m/s2 = 500,000 N. Weight is a force, since it is mass times the acceleration due to gravity. If you lived on a different planet, your weight would be different, but your mass would be the same. Work out an example using your own weight, similar to the example worked out in the book on p. 80. You need to know that the acceleration due to gravity "ag " on the surface of the Earth is 9.8 m/s2. You also need your mass in kilograms, which is your weight in pounds, divided by 2.2. Now you can figure out your weight in Newtons. Gravitation: Let's go back to the idea of planetary orbits. As we said above, orbits are curved, so the Sun must be exerting a force on the planets. How big is the force? Newton's Law of Universal Gravity states that every body in the universe is attracted to every other body in the universe. The equation is F = Gm1m2/r2 where m1 = the mass of one body m2 = the mass of the other body r = the distance between the centers of the bodies G = the Gravitational Constant = 6.67 x 10-11 Nm2/kg 2. Note that to use this value of the constant G, we must use the meters for the distance and kg for the masses. The force is then given in Newtons. What this equation means in words is that The more massive either object is, the greater force between them. (That's why you feel the gravity from the Earth, but not from the person sitting next to you.) The farther the objects are from each other, the less is the force between them. (That's why you don't feel the gravity of the Sun, even though it is much more massive than the Earth.) Remember that the force is truly between the two objects. Both experience the same size pull. But if they are very different in mass, the small one will experience most of the acceleration, because of Newton's second law. This formula is universal: it applies to everything in the universe (including you). For example, what is the force a person with a mass of 60 kg feels from the Moon? The Moon is 3.85 x 108 m away and has a mass of 7 x 10 22 kg. F = Gm1m2/r2 F = 6.67 x 10 -11 N m2 / kg2 x 7 x 1022 kg x 60 kg/(3.85 x 108 m)2 F = 1.9 x 10-3 N Imagine yourself sitting next to someone in a classroom. Make some reasonable assumptions about your masses and distance, and calculate the gravitational force between you. This should turn out to be much smaller than the force of gravity between you and the Earth, which is several hundred Newtons. (See the calculation in the book on p.80, for a 50-kg. diver. This person is slender, but it will give you a ballpark figure.) Gravity and Orbits: There is no limit on the distance gravity acts. The force of gravity extends through space for all objects, and that's how gravity can pull planets and moons into orbits. Remember that according to Newton's first law of motion, things move in straight lines unless acted on by a force. If the force acts perpendicularly to the motion, then the object's direction will change rather than its speed. Consider a cannon firing from a high mountain top. As the speed of the cannonball is increased, it falls farther away from the cannon. This is what Figure 4-20 shows. If the speed is fast enough (roughly 8 km/s) and assuming there is no atmospheric drag, then the Earth's surface will curve away as fast as the cannonball falls. The cannonball will go into orbit, and return to its starting point. If the speed is a little faster, the cannonball will pull away from the Earth for a while, then it will fall back and return to its starting point. It is in an elliptical orbit, which moves fastest when it is closest to the Earth. This is just like Kepler's 1st and 2nd Laws of Planetary Motion. If the speed is faster still (around 11 km/s), it won't return. It has "escaped" from the Earth's gravity. Kepler's 3rd Law of Planetary Motion can also be generalized from Universal Gravity. Read the example in Box 4-4. Here we see that Newton found a way to rewrite something very similar to Kepler's third law. (Notice that both equations use P2 and a3 in similar ways.) The most important difference is that while Kepler's version applied only to the planets orbiting the Sun, Newton's version applies to any two bodies orbiting anywhere in the universe. (This pretty much describes the difference between Kepler's and Newton's work in general.) The difference between the two equations is the stuff in square brackets. Look at it, and think about what is there. The Greek symbol pi (p ) and G are just numbers whose value we know (or could look up), so the unknown in the square brackets is just the sum of the two bodies' masses. Therefore, if we know the size and orbital period, we can find the total mass. The cute trick here is realizing that for a big planet and a tiny satellite, the total mass is little different from the mass of the planet alone. Your book uses the example of Jupiter's tiny moon Io orbiting massive Jupiter. Follow that example, then try one for yourself: What is the mass of the Earth? The Moon orbits the Earth is 27.3 days (=27.3 x 24 hrs. x 60 min. x 60 sec. = 2.36 x 106 sec.). The semimajor axis of the Moons orbit is 3.85 x 105 km (=3.85 x 108 m). You can check your answer in the tables in the back of the book. This technique is incredibly powerful. When astronomers look at the universe, they find many examples of bodies orbiting each other. This is how we measure masses of planets, moons, stars, galaxies, and even clusters of galaxies. I know by now you've slogged through a lot of hard stuff in this chapter's notes, but from an astronomical point of view, we've got two ideas that are really worth the effort: Gravity is responsible for orbits. From orbits we can measure masses. When we start to look at the planets of the solar system as worlds--real places we would like to know about--it will turn out that a planets orbit and mass are involved in the answers to just about every question we might be interested in: Is it hot or cold there? Wet or dry? Is there any atmosphere? Are there mountains and valleys? Volcanos? Impact craters? Is the world geologically active or is it geologically dead? Why? Would it be a nice place to live? Your (and astronomers') hard work to figure out these things will be well worth it!! Tides are caused by gravity and the motions of the Earth and Moon: Most of the Earth's oceans have 2 high and 2 low tides a day. Tides are typically related to the position of the Moon in the sky. High tide occurs when the Moon is overhead or underfoot. Let's look at how this works: The Earth's pull keeps the Moon in orbit. From Newton's 3rd Law of motion, the Moon must also be pulling on the Earth. One side of the Earth is nearer to the Moon than the other, so the pull of the Moon's gravity is not felt with the same strength everywhere on Earth On the side facing the Moon, the Moon's pull is stronger. On the side facing away from the Moon, the force of gravity is weaker. Look at Figure 4-23 and animation 4-7 on the Universe CD-ROM that came with your textbook, or at the publisher's web site . These show you how chunks of matter at slightly different distances feel differential pulls. From the point of view of the blue ball in the middle, it seems that the red and yellow balls on the end are pulled away to both sides. If the balls were connected by springs, the springs would get stretched. Figure 4-25 applies this to the Earth. The combination of forces gives the Earth two bulges, one on the side facing the Moon and one on the side away from the Moon. The solid part of the Earth is fairly rigid, so it doesn't bulge out much. But the water in the oceans can flow around to fill in the bulges. This produces the high tides. As the Earth spins, these bulges stay pointing towards and away from the Moon. This means that to an observer on earth, the bulges seem move around the Earth. A better way of putting it might be that any point on Earth spins past the stationary bulges. The shapes of the continents and shorelines strongly affect the motion of tide water, but still, an observer on a typical point on a coastline spins past both bulges and both low points each day. The Sun exerts a tide on the Earth for exactly the same reasons. The Sun's tide is smaller because the Sun is so far away that the pulls on the two sides of the Earth are more closely equal. Figure 4-25 shows you how the Moon's and Sun's tidal pulls can line up and reinforce each other (spring tide) or be at right angles and diminish the difference between the highs and lows (neap tide). Tidal evolution has shaped the worlds of the solar system: Tides will turn out to be very significant in this course. Whenever tides occur, the rock, liquids, and gases of the world are squeezed and stretched repeatedly. Some of the energy of motion (spin or orbits of bodies) gets turned into heat by friction, and the friction slows down the orbit or spin. In the past the Moon may have rotated much faster than its orbit around the Earth. The Moon's faster rotation would have caused the tidal bulge to rotate around the Moon. The friction of this motion would have slowed the Moon's spin until it always kept the same face toward the Earth. (Review Figure 3-4.) Tidal evolution has caused most of the moons in the solar system to keep the same face towards their planets. This may also be partially responsible for Mercury and Venus having very slow rotation rates. The same tidal slowing is causing the Earth's spin to slow down. Eventually the Earth will always keep one face towards the Moon. As the Earth's spin slows, the Moon's orbit is getting larger. That's because the Earth's tidal bulge is forcing the Moon into a higher orbit. (Figure 9-19) In general, any time a planet spins faster than its moon orbits, the moon will pull slowly away from the planet, and the planet's spin will slow down. There are a few cases in which the moon goes around the planet faster than the planet spins. In this case, the moon tends to drop down into towards the planet. One example is Mars. Its moon Phobos orbits in just over 8 hours. Mars spins once every 24 hours. (Figure 12-23) Phobos is slowly spiraling toward Mars. At some point in the distant future it will crash into the planet. In the early history of the solar system this may have been important for many planet/moon systems, especially Earth/Moon, Pluto/Charon, and Neptune/Triton. Tidal heating is currently important is on Jupiters moons Europa and Io (and maybe Ganymede), and possibly the Earths moon. (Figures 14-8, 14-11) Starry Night companion STARRY NIGHT COMPANION Go to Table of Contents Starry Night Companion Your Guide to Understanding the Night Sky Using Starry Night Written by John Mosley Edited by Mike Parkes Foreword by Andrew L. Chaikin SPACE.com www.space.com www.starrynight.com support@starrynight.com 284 Richmond St. E. Toronto, ON M5A 1P4, Canada (416) 410-0259 1993-2000 SPACE.com Incorporated. All rights reserved. SPACE.com, the SPACE.com logo, Starry Night, the Starry Night logo, and LiveSky are trademarks of SPACE.com, Inc. Cover photo composite Dennis di Cicco and Terence Dickinson ISBN 1-894395-04-2 Printed in Canada. CONTENTS Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Section 1 - Astronomy Basics 1.1 The Night Sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.2 Motions of the Earth . . . . . . . . . . . . . . . . . . . . . . . . 33 1.3 The Solar System . . . . . . . . . . . . . . . . . . 43 Section 2 - Observational Advice 2.1 SkyWatching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 2.2 Co-ordinate Systems . . . . . . . . . . . . . . . . . . . . . . . . 63 Section 3 - Earths Celestial Cycles 3.1 Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.2 Revolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.3 Precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Section 4 - Our Solar System 4.1 The Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 4.2 Planets, Asteroids & Comets . . . . . . . . . . . . . . . . . 131 Section 5 - Deep Space 5.1 Stars and Galaxies . . . . . . . . . . . . . . . . . . . . . . . . . 157 5.2 About the Constellations . . . . . . . . . . . . . . . . . . . . 173 5.3 Highlights of the Constellations . . . . . . . . . . . . . . . 189 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Appendix A - The Constellations . . . . . . . . . . . . . . . . . 213 Appendix B - Properties of the Planets . . . . . . . . . . . . . 219 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Blank Page FOREWORD If the sky could speak, it might tell us, Pay attention. That is usually good advice, but its especially valuable when it comes to the heavens. The sky reminds us that things are not always as they seem. It wasnt too long ago that we humans believed the Earth was the center of it all, that when we saw the Sun and stars parade across the sky, we were witnessing the universe revolving around us. It took the work of some bold and observant scientists, including the Polish astronomer Nicolaus Copernicus, to show us how wrong we had been: Instead of the center of the universe, Copernicus said, our world is one of many planets and moons circling the Sun in a celestial clockwork. Some 250 years later the Apollo 8 astronauts became the first humans to see the truth of this with their own eyes. As they made historys first voyage around the moon, they looked back at the Starry Night Companion 8 Earth, whose apparent size diminished until they could cover it with an outstretched thumb. In that moment, they knew on a gut level that Copernicus was right. Most of us, though, have not had the privilege of such a view. Standing under a canopy of stars on a clear night, weve had to take on faith what astronomers have told us. Starry Night, whose many features are described in this book, changes all that. John Mosleys clearly written and engaging text is filled with details about this remarkable desktop planetarium, which allows you to make virtual journeys into the cosmos. Want to see why the moon changes in appearance during a month? Starry Night lets you stand on an imaginary vantage point in deep space and see the moons motion around Earth, watching as the lunar nearside goes from darkness into sunlight and back again. Farther from home, you can watch Jupiters family of moons revolve like a solar system in miniature. You can ride on a comet as it makes its decades-long trek around the Sun. You can even journey back in time, to see the sky as it looked when Galileo turned his first telescope on the heavens. Or, you can peek into the future, to get an advance look at solar eclipses, lunar eclipses, and other skyshows. A Starry Night Companion, then, offers a tour guide to the heavens. In the these pages, youll do something Copernicus could only dream of: You will learn your way around the universe. Youll come to know it as a place filled with wonders, from red giant stars to glowing nebulae. Youll experience what it would be like to view the sky from hundreds of light years away. And like the astronauts, youll return to Earth with a new perspective on our planet and its place in the cosmos. Best of all, youll enrich your experience of the wonders above our heads. The sky is there for us to enjoy, and that enjoyment is 9 Foreword only increased when we understand what we are seeing. When we stand under a dome of stars, we can see it with the benefit of centuries of detective work that have unraveled the heavens secrets. And we can ponder the mysteries that remain for us to solve, if we only pay attention. Andrew Chaikin Executive Editor, Space and Science SPACE.com August, 2000 Blank Page INTRODUCTION The sky is enormous, distant, and filled with mysterious things. It doesnt resemble anything we encounter in our daily lives. There is movement and change in the sky, but generally at a rate too slow for us to notice. To many, the sky is intimidating. It may seem inaccessible. But it is superbly fascinating. You bought this book and software package because you would like to get to know it better. Welcome - the adventure is about to begin. Two hard facts about the sky are that (1) changes in it happen slowly, and (2) we cannot cause it to change. We cannot turn it and look at it from a different angle; we cannot speed up the rotation of the earth or the motion of the planets through the constellations; we cannot cause an eclipse to happen; we cannot view the sky from another place on earth (unless we actually go there); we cannot see what the sky looked like long ago or in the distant future. We are stuck in the here and now. That is the value of this book and software package. Starry Night lets you manipulate the sky. You can move backward and Starry Night Companion 12 forward through time as far and as fast as you wish; you can move to any spot on the earth and to any planet; you can see imaginary lines in the sky (such as the paths of the planets and constellation boundaries); you can discover future events and watch them before they actually happen, or replay famous events from the past. You can take control of the sky. As you go, you will gain a far better understanding of how the sky works than you can as a passive observer. Then, when you actually step outside on a clear night, you will have a greater understanding of what you see. It will be fun. Starry Night lets you stargaze on cloudy nights and even during the daytime, but there is no substitute for stepping outside at night and seeing the real thing. Starry Night will help you appreciate what you see when you are outdoors on a starry night, but dont forget to actually go outside and look up! How to Use This Book Starry Night Companion is organized into five sections. Starry Night example files will be used in almost every section to complement the material in the text. These files can be opened by clicking the Go menu in Starry Night and then choosing Companion Book. The files are subdivided by chapter. Just click on the file you are interested in to launch this file. If there is no Companion Book item in your Go menu, you are using an older copy of Starry Night, and should update by visiting www.starrynight.com The first section covers the essentials of understanding the night sky. Several step-by-step exercises let you get familiar with Starry Night and show how it can help you learn about astronomy. Newcomers to the hobby may want to read section one and then 13 Introduction spend some time using Starry Night and making their own observations, before returning to the rest of this book. The next four sections go into more detail. Section two will help you get the most out of your observing, no matter what you are looking at. It has observational tips, hints on choosing binoculars and telescopes, and a good description of the various astronomical co-ordinate systems. Section three covers the three different motions of the earth which account for much of the variation we see in the sky. Section four looks at other objects in our solar system, including our moon and the nine planets. Finally, the fifth section takes you into the deep, describing stars, galaxies, and more. Blank Page Section 1 Astronomy Basics THIS SECTION of Starry Night Companion will give you a quick overview of the basics of astronomy. Chapter 1.1 has general information, while chapter 1.2 looks at the motion of the earth and chapter 1.3 introduces the other bodies in our solar system. 1.1 THE NIGHT SKY The Constellations Stepping outdoors at night in a dark place far from the bright lights of the city, we are amazed at the number of stars we can see. Bright stars and dimmer ones, tightly knit clusters with many points of light and dark patches with nothing at all. Stars, stars everywhere, stretching off to the horizon in all directions, as far as the eye can see. But which one of those bright points is the North Star? Is that reddish dot Mars or a relatively cool star? And isnt the International Space Station up there somewhere tonight? Wouldnt it be nice to have a map? In fact, the sky has been mapped. Astronomers divide the sky into 88 different non-overlapping areas called constellations. You can think of the constellations as the countries on the surface of the sky. Each constellation is an area of the sky. Just like countries Section 1 - Astronomy Basics 18 on Earth, some constellations are bigger than others. Everything inside the boundaries of a constellation is considered to be part of that constellation, regardless of its distance from Earth. A constellation is actually a wedge of the universe with Earth at its center that extends to the farthest reaches of space. The brighter stars in each constellation make a unique pattern, and we look for this pattern to identify the constellation. To help make this identification easier, people through the ages have connected the dots of stars to draw recognizable patterns called constellations, which they then named. Sometimes the pattern actually looks like the object after which it is named; more often it does not. One point of confusion among newcomers is that the boundaries and the connect-the-dot stick figures share the same constellation name. If you read that the sun is in Scorpius, the The constellation Crux has the appearance of a cross, but Pisces bears little resemblance to a fish. Constellation maps of the northern (left) and southern celestial hemispheres. 19 The Night Sky scorpion, it means that the sun is inside the official boundaries of the constellation Scorpius, but it is not necessarily inside the stick figure that outlines a scorpion. Starry Night can show you the constellation boundaries, stick figures, or both. The constellations are the fundamental units of the visual sky, yet they are imaginary. They were invented. People created them long ago - in some cases in prehistoric times - and often for reasons that we might think strange today. One constellation leads to another, so to speak. Use easilyrecognizable star patterns to find more obscure ones. The two stars at the end of the bowl of the Big Dipper, for example, point to the North Star and to the Little Dipper, while the five stars of the handle of the Big Dipper point to the star Arcturus in Bootes. The three belt stars of Orion point west to Aldebaran in Taurus and east to Sirius in Canis Major. It is easy to draw lines and arcs from star to star and star hop from constellations you know to those you are still learning. As in so many endeavors, dont bite off too much. Dont try to learn the entire sky on your first night out. Mars, Jupiter, and Saturn are all considered to be in Aries, though they are outside its stick figure. The belt of Orion is sand- sandwiched wiched between the bright stars Sirius and Aldebaran. The Big Dippers Merak and Dubhe make a straight line leading to Polaris. Section 1 - Astronomy Basics 20 Begin by learning the major constellations and then fill in the small and obscure ones as the need and challenge arises. A curious feature of the constellations is that once you know them, they are hard to forget. Like riding a bicycle, once you know them you tend not to forget them, and they will remain with you for a lifetime. If you know the constellations, the sky is never an unfamiliar place and you will be at home under it wherever you go. The constellations are described in much more detail in section 5 of this book. Co-ordinate Systems Although the constellation boundaries are a good starting point for describing the approximate location of an object in the sky, astronomers need a more precise method of specifying an objects position. There are several different co-ordinate systems in common use, and the most intuitive is the horizon co-ordinate system, also known as the Alt-Az system Alt is short for altitude, which is an objects height above the horizon in degrees (0 to 90). An object with an altitude of 0 is right along the horizon, while one with an altitude of 90 is directly overhead in the sky. The imaginary point in the sky directly overhead is known as the zenith. Knowing the altitudes of objects at different times will help you plan your observing sessions. The best time to observe something is when it has an altitude between 20 and 60. This is because objects (especially planets) near the horizon can be blurred by intervening air currents, while objects with an altitude greater than 60 are tough to look at without straining your neck! Avoid observing planets near the horizon. 21 The Night Sky Az stands for azimuth, an Arabic word (as is altitude) for the position along the horizon; bearing is an alternative navigational term. Azimuth is measured in degrees from north, which has an azimuth of 0, through east (which has an azimuth or a bearing of 90), through south (180) and west (270). The local meridian is the imaginary line which divides the sky into an eastern and a western half. It extends from the southern point on the horizon through the zenith to the northern point on the horizon. The half of the meridian line which is south of the zenith has an azimuth of 180, while the half north of the zenith has an azimuth of 0. Once you have the altitude and azimuth of an object, you know where to find it in the sky. A compass is helpful in determining the azimuth of your viewing direction (compasses point to magnetic north, which is not exactly the same as true north, but the two directions are practically identical unless you are observing from a far northern latitude). Once you have your bearing, it is just a matter of looking up to the proper altitude. With practice, you will quickly learn how high above the horizon is 10, how high is 30, and so on. Starry Night can show you the altitude and azimuth of any object at any time (just double-click on the object to bring up its Info Window with this information) and can also mark the zenith and nadir points and/or the meridian line (choose the appropriate option from the Guides menu). Although the horizon co-ordinate system is the easiest to understand, it is not necessarily the most useful system. This is because it is a local system, and the co-ordinates depend on your personal location. Jupiter, for example, will have one set of horizon co-ordinates for an observer in California, a second set of coordinates for an observer in Texas, and a third for an observer somewhere else. To make things worse, the co-ordinates change constantly over time as the sky rotates. For these reasons, stars and other objects in the sky are more commonly identified using the Section 1 - Astronomy Basics 22 equatorial co-ordinate system. This system gives one unique set of co-ordinates for an object, which is the same anywhere on Earth. The equatorial system is described in chapter 2.2. Angles in the Sky Astronomy is full of angular measures, and it is very handy to have a conceptual feeling for what these angles mean. A circle is divided into 360, so the distance from the horizon to the zenith is one quarter of a circle or 90. It is also 90 along the horizon from one compass point to the next (east to south, for example). Familiar constellations will help you visualize smaller angles. The distance from the end star in the handle of the Big Dipper to the end star in the bowl is 25. The distance between the two end stars in the bowl is 5. The sides of the Great Square of Pegasus average 15 in length. The distance from one end of the W of Cassiopeia to the other is 138. The distance from Betelgeuse to Rigel in Orion is 19, and the length of Orions belt is just under 3. Your hand is a portable angle-measurer. The width of your clenched fist held at arms length is about 10 (people with shorter arms generally have smaller fists and the general rule holds). The width of a finger at arms length is about 2. The angular diameter of the moon is only 1/2, although most people would guess it is much larger. Try it yourself. Block out the full moon with your little finger held at arms length. The field of view is an astronomical term which often confuses newcomers to astronomy. It is the angular width of the patch of sky you can see through your optical instrument, whether that instrument is the naked eye, a pair of binoculars or a telescope. Field of view is usually expressed in degrees for binoculars and arcminutes for telescopes. An arcminute is 1/60 of a degree. An even smaller unit, an arcsecond, is 1/60 of an arcminute. 23 The Night Sky The entire sky from horizon to horizon encompasses 180, but we see only part of it at once, and with binoculars and telescopes we see very little at a time. With the naked eye, we have a field of view of about 100, which is the field of view you see when you open Starry Night. Binoculars typically give fields of view of 5 to 7, which is a large enough portion of the sky to see the bowl of the Big Dipper or the belt of Orion, but not an entire constellation. A telescope with a low-power eyepiece typically gives a field of view of 30 arcminutes (expressed as 30), which is the equivalent of 1/ 2. A high-power eyepiece on the same telescope may give a field of view of 10 arcminutes (or 1/6). It is difficult to find objects with a high magnification telescope because you can see only a very small part of the sky at one time. Starry Night displays your field of view and can also display circular outlines which represent the fields of view of different optical instruments. The relative views of Orions belt as seen through 7 X 50 binoculars (7 field of view) and a low-power telescope eyepice (30' field of view). Stars Gaze upwards on a clear night and you see - stars! A lot of stars, if you are lucky enough to be in a dark place. How many is a lot? From a dark location a person with good eyesight can see about 2,000 stars at any moment, or about 6,000 if he could see the entire sky. Section 1 - Astronomy Basics 24 It is useful to have a system for classifying the brightness of stars, and the one we use has been around for thousands of years. When compiling his star catalog in about 150 BC, the astronomer Hipparchus devised the scheme still used today. He divided the naked-eye stars into six magnitude groups, with lower numbers designating brighter stars. First magnitude stars are the brightest and sixth the dimmest that most people can see without a telescope. It is counter-intuitive for the brighter stars to have a smaller magnitude number, but the scheme has been used for so long that astronomers have gotten used to it. Modern astronomers have recalibrated Hipparchus scheme to put it on a sound mathematical footing, and magnitudes are now expressed as decimals. The brightest star, Sirius, is now assigned a magnitude of -1.4. The planets can be even brighter, Jupiter reaching -2.9 and Venus -4.5. The full moon is magnitude -12.6 and the sun is -26.7. The magnitude of the dimmest object you can see is called the limiting magnitude. With the naked eye on a clear night, the limiting magnitude is about 6th magnitude, but with light pollution it can be much higher. Binoculars and small telescopes will see down to 9th magnitude, a large amateur telescope to 14th magnitude (the magnitude of Pluto), and the Hubble Space Telescope reaches to 30th magnitude. Starry Night can show you the magnitude of any object. The stars of Perseus visible with a limiting magnitude of 6, and a limiting magnitude of 3. 25 The Night Sky Astronomical Distances Most people know that distances in outer space are quite large- astronomical, so to speak. Our normal measurements of length become meaningless in these situations, and astronomers have invented two new units of length which serv e different purposes in astr onomy. The astronomical unit (AU) is the average distance from Earth to the sun. This equals 150 million kilometres (93 million miles). It is used primarily to describe distances to objects within the solar system. For example, Jupiter is about 5 AU from Earth on average, while Neptune is about 30 AU away. When talking about the distances to the stars, even the astronomical unit is not large enough. The nearest stars are about 250 000 AU away, which is a remarkable 37 500 000 000 000 km ( 24 000 000 000 000 miles)! For distances to stars and other deep space objects, astronomers use the light year year. A beam of light (or radio waves or any other sort of electromagnetic radiation) travels at the speed of light, which is 299,792 kilometers (186,282 miles) per second. That is fast enough to travel seven times around Earth in one second or to the moon and back in three seconds. At this incredible rate, a beam of light travels 9,460,540,000,000 kilometers (5,878,507,000,000 miles) in one year - and that distance is one light year. A light year is a unit of distance distance, not time time, and it is approximately 9-1/2 trillion kilometers (6 trillion miles). Section 1 - Astronomy Basics 26 Each magnitude step represents a difference in brightness of 2-1/2 times; for example, a 3rd magnitude star is 2-1/2 times as bright as a 4th magnitude star. (The actual ratio is 2.512, which is the 5th root of 100; the difference between a 1st and 6th magnitude star is 100 times.) The Hubble Space Telescope can photograph stars 24 magnitudes or 4 billion times fainter than the dimmest stars your eye can see. The magnitude of a star depends on two things: the amount of light it gives off and its distance from Earth. The stars which can be seen from Earth range in distance from about 4 light years to several thousand light years, making it impossible to determine how intrinsically bright a star is by its magnitude alone. Learning to See Although sixth magnitude is the theoretical naked-eye limit for most people, you may have trouble seeing objects this faint at first. It takes practice to learn to sing a song, throw a baseball, or look at a small, faint object through a telescope - or at least it takes practice to do it well. You must learn how to see. Slow down, and let your eye absorb the image. Astronomical objects are small and contrast is low, so details do not spring out. The most important thing to do is to relax, linger at the eyepiece, and let an image slowly accumulate on your eye. The greatest amateur astronomer of all and the discoverer of the planet Uranus, William Herschel, said, You must not expect to see at sight. Seeing is in some respects an art which must be learned. Many a night have I been practicing to see, and it would be strange if one did not acquire a certain dexterity by such constant practice. Go slow, and take the time to see. 27 The Night Sky The Night Sky Starry Night Exercise This exercise is designed to complement the concepts discussed in this chapter. The instructions are written for Starry Night Backyard, but will also work with Starry Night Pro. If there are differences in instructions between the two versions, this will be explained in the exercise.You should open Starry Night on your computer before beginning the exercise. As mentioned earlier, the sky is divided into different constellations. Starry Night lets you display the constellations in several different ways. 1) Open a new window by selecting File | New. 2) Choose Go | Viewing Location to bring up the Viewing Location window. 3) Click the Lookup button and select Chicago from the list of cities. Click the OK button. The co-ordinates for Chicago should now appear in the Viewing Location window. Make sure that the box marked DST is not checked. Click the Set Location button to change your viewing location to Chicago. 4) Change the date to Dec.1, 2000 and change the time to 10:00 PM. Press the Stop button in the time controls to keep time fixed at 10:00 PM. 5) Select Constellations | Boundaries (Guides | Constellations | Boundaries on Pro. All constellation options in Pro require this extra step) to draw the constellation boundaries on the screen and then select Constellations | Labels to label them. Select Constellations | Constellation Settings and press the Labels button in the Constellation Settings window. In the Label Settings window which comes up, make sure the option Both is selected (under the heading Constellation Label Options). This displays the common and astronomical names of the constellations. Press the OK button to close these two windows. Scroll around the sky using the mouse and notice the different sizes of the constellations and their irregular shapes. 6) Starry Night gives you several different ways to draw the stick figures of the constellations. Select Constellations | Astronomical to turn on the common stick figures (on Starry Night Pro, choose Constellations | Stick Figures). You can see that all the stick figures stay within their constellation boundaries. Again, scroll around the Section 1 - Astronomy Basics 28 sky and note that few figures bear any resemblance to their names, although Orion (in the southeast) does resemble a hunter with bow, and the Great Bear (in the northeast) is shaped like an animal. 7) Select Constellations | Reys (on Starry Night Pro, you need to choose Guides | Constellations | Constellation Settings and click the button marked Reys) to display the constellation figures popularized by H.A. Rey. He designed these figures so that the pictures they made corresponded with the constellation names. If you view the real night sky, however, you may have trouble matching the star arrangements with Reys figures! 8) As an example of how you can use the constellations to look for an object, suppose you want to observe the star cluster M52. Open the Find dialog by choosing Edit | Find (Selection | Find on Starry Night Pro) and type in M52 in the field marked Name Contains. Make sure that the checkbox marked Magnify for Best Viewing of Found Object is not checked. Click the Find button to have Starry Night center and mark the object. You can see that M52 is in Cassiopeia. Select "Constellations | Labels" to turn off the labels, and notice the distinctive W figure which Cassiopeia makes in the sky. This W is one of the easiest star patterns to find in the night sky. You can see that the rightmost line of the W (the line that connects the stars Schedar and Caph) points to the cluster. This will help you point your binoculars on M52 when you go observing (note that M52 is too faint to see with the naked eye). 9) Now turn off all the constellation indicators. Select Constellations | Boundaries to turn off the boundaries, and Constellations | Reys (Constellations | Stick Figures in Starry Night Pro) to turn off the stick figures. You learned in this chapter that another way to find an object is to know its co-ordinates in the Horizon system: its altitude and azimuth. 1) Starry Night shows you the altitude and azimuth of the center of the screen when you are scrolling around the sky. This is called your gaze information. In Starry Night Backyard, the gaze information appears along the bottom center of the screen. In Starry Night Pro, it will appear in one of the four corners, depending on how you have set up your preferences. To pull up the gaze information at any time, just hold down the left mouse button. 29 The Night Sky 2) Remember that altitude measures how far above the horizon an object is located. Scroll up in the sky until the zenith (marked in red when you are scrolling or holding down the left mouse button)) is near the center of the screen. By looking at the gaze information, you will see that the altitude is near 90 degrees. Similarly, if you scroll down in the sky so that the horizon is near the center of the screen, you will notice that the altitude is near 0 degrees. 3) Azimuth is measured in degrees from North. If you scroll around so that the compass marker for North is near the center of the screen and then hold down the left mouse button, you will see that the azimuth is close to 0 degrees (or just less than 360 degrees-because the sky is divided into 360 degrees, the counter resets at 360). Likewise, if you move around on screen so that the East compass point is near the centre of the screen, the azimuth will be near 90 degrees. 4) Now, we will use an objects Alt/Az co-ordinates to find it in the sky. You are told that Sirius has an altitude of about 10 degrees, and an azimuth of about 124 degrees (remember that Alt/Az co-ordinates are only valid for a specific time and will change as the star moves in the sky-these co-ordinates are valid for 10 PM on Dec.1, 2000 from Chicago). Use your understanding of how these co-ordinates relate to the horizon and the direction markers to locate Sirius, the brightest star in the sky. Remember that at any time, you can hold down the left mouse key to find the Alt/Az co-ordinates of the center of the screen. 5) Starry Night can also tell you the Alt/Az co-ordinates of an object at any time. Open the Find dialog and find Polaris, also known as the North Star. Starry Night will then center on Polaris and identify it. Double-click on Polaris to pull up its information window (if no window opens, you didnt have the cursor positioned directly over the star. You should see the cursor icon change from a hand to an arrow before you double-click on the star). If you are using Starry Night Pro, click the Position tab. Make sure the dropbox in this window reads Local (Alt/Az). Polariss altitude and azimuth are shown just beneath the dropbox. Note that the altitude is approximately equal to your latitude (the latitude of Chicago is about 41.5 degrees N) and the azimuth is very close to 0 or 360 degrees. This is always the case for the North Star-it is the one star in the sky which doesnt appear to move over time, so its Alt/Az co-ordinates remain constant. Close the Info Window. Section 1 - Astronomy Basics 30 6) Select Guides | Celestial Poles (Guides| Equatorial | Celestial Poles in Starry Night Pro). This will turn on markers for the North and South Celestial Poles. The North Celestial Pole has an altitude exactly equal to Chicagos latitude, and an azimuth of 0. Note how close it is to Polaris. You may want to choose Edit | Select None (Selection | Select None in Starry Night Pro) to get rid of the label for Polaris, because it will overlap the label for the North Celestial Pole. You will now use Starry Night to get a better understanding of the different angular sizes of objects in the sky. Starry Night shows you a field of view approximately equal to what you can see with your eyes, about 100 degrees. Your current field of view is always displayed near the two zoom buttons. 1) Starry Night allows you to find the angular separation of any two objects. Open the Find dialog and find Alkaid. This will center the end star in the handle of the Big Dipper. 2) Now position the cursor over this star. Wait for the cursor icon to change from a hand to a pointer. Click and hold the mouse button and then move the mouse. A line will appear on screen, drawn from Alkaid to the current position of your cursor. The angular separation of the two points is shown in degrees, arcminutes and arcseconds. 3) Continuing to hold the mouse button down, move the cursor over the star at the end of the bowl of the Big Dipper, which is named Dubhe. You can see that the angular separation of the stars at the ends of the Big Dipper is just over 25 degrees. You can find the angular separation of any two objects this way. 4) Open the Find dialog and find Andromeda Galaxy. This will center on the Andromeda galaxy. It is almost invisible in your current field of view. (make sure the Magnify for best viewing checkbox is still turned off or the nebula will fill the screen). 5) Select Guides | Field of View Indicators | 7 X 50 Binoculars. This draws a circle over the centre of the screen indicating the patch of sky you would see if you were looking through binoculars of this common type. 6) Use the zoom button to zoom in on the Andromeda galaxy. Zoom in until the circular outline covers almost the entire screen. Notice how the field of view changes as you zoom in. Now you can see how the 31 The Night Sky size of the galaxy compares to the field you see through your binoculars. The binoculars have a field of view of about 7 degrees across (the number shown near the zoom buttons will be a bit larger at about 10 degrees, because it is the angular width from the left side of the screen to the right side). The angular width of the Andromeda galaxy is just under 2 degrees. You can see that the galaxy still appears quite small when seen through binoculars of this type. 7) Press the square button to the right of the zoom buttons. This restores your field of view to 100 degrees. 8) Select Guides | Field of View Indicators | 7 X 50 Binoculars to remove the circular outline from the screen. To conclude this exercise, you will learn a bit more about magnitudes. When you are looking at a regular 100 degree field of view, Starry Night shows all stars brighter than magnitude 5.7 (5.5 in Starry Night Pro), which matches approximately what you could see in a dark sky. As you zoom in, the limiting magnitude automatically increases to show you dimmer objects. If you hold down the left mouse button, the limiting magnitude is shown along the top of the screen. 1) Open the Find dialog and find Sirius. 2) Bring up the info window for Sirius. The stars magnitude is shown in the window that appears (on Starry Night Pro, you need to make sure the Info tab in this window is marked) . For Sirius, the magnitude is -1.47. 3) By bringing up info windows for other nearby stars, you can see how magnitude relates to brightness. For example, the three stars in Orions belt (above Sirius, slightly to the right) all have magnitudes between 1.6 and 2.3, about 3 to 4 magnitudes dimmer than Sirius, while Betelgeuse (the bright star at the top of Orion) has a magnitude of 0.4, about 2 magnitudes dimmer than Sirius. 4) Changing the limiting magnitude affects the number of stars you see. Select Sky | Small City Light Pollution (Sky | Light Pollution | Small City in Starry Night Pro). The limiting magnitude is now 4.5. Note how many fewer stars are visible. 5) Select Sky | Large City Light Pollution (Sky | Light Pollution | Large City in Starry Night Pro). The limiting magnitude is now 3.0 and only the brightest stars are visible. 6) Which of the three views do you think best matches the night sky you normally see? Select Sky | No Light Pollution (Sky | Light Pollution | None in Starry Night Pro), then turn on the constellation names and stick figures. Scroll around the sky and find Ursa Minor, also known as the Little Dipper. The seven majors stars that make up the Little Dipper have a wide range of magnitudes. Polaris and Kochab are fairly bright with magnitudes around 2, while at the other extreme, Eta Ursae Minoris has a magnitude of about 5. The other 3 stars have magnitudes in between these extremes. Find the magnitudes of all six stars (by bringing up their Info Windows) and write them down. The next time you are observing, determine which ones you can see with the naked eye. Your limiting magnitude is somewhere between the magnitude of the dimmest star in the Little Dipper that you can see, and the magnitude of the brightest star in the Dipper that you cannot see. If you can see all the stars in the Little Dipper, count yourself lucky! 1.2 MOTIONS OF THE EARTH There is movement in the sky. Not only do the sun, moon, planets, comets, and asteroids move against the background of stars, but the sky itself moves. These changes happen because of the motions of the earth as it spins on its axis and orbits the sun. As the night passes and as the seasons change, we face different parts of the universe and see different stars and constellations. Rotation of the Sky Many celestial motions are too slow to be noticed over so short a period as a night or even a month, but the nightly rotation of the sky happens on a scale that, with a little patience, we can experience while we gaze upward. The skys rotation is shown dramatically in long time-exposure photographs centered on the North Star, which show the motion of stars as circular trails of different sizes centered on the skys North Pole. We speak of the sky rotating overhead, although we know that it is the earth that is turning. The earth makes Section 1 - Astronomy Basics 34 one rotation a day, spinning from west to east, which causes the sky to turn from east to west. We speak of the sun rising in the morning, although we know that it is the earth that turns towards it, making the sun appear to rise above the horizon. The illusion is so convincing that it wasnt until the time of Copernicus in the 16th century that people accepted that the earth does indeed turn on its axis. Proving that the Earth Spins How would you demonstrate that the earth spins rather than the sky sky? No simple visual demonstration existed until the Frenchman Jean Foucault hung a massive iron ball from the high dome of the Pantheon in Paris in 1851 and set it swinging. Foucault demonstrated that this pendulum appears to slowly change the direction of its swing relative to the ground beneath. Since the pendulum does not feel the orientation of the building it is attached to, the earth is free to rotate under it without affecting the direction of its swing. The pendulum feels the sum of the gravitational pull of the rest of the universe and maintains a constant orientation relative to the distant stars. Foucault Pendulums are found today in planetariums and science museums. Long time-exposure photo showing star trails that arc as the earth spins. 35 Motions of the Earth Annual Motion of the Sun The daily rotation of the earth on its axis is one fundamental motion of the earth (and of the sky). The second is the annual revolution of the earth around the sun. Until the 16th century, it was taken as a matter of faith that the earth does not move and is the center of all creation. Ancient Greek musings contrary to this view were taken as mere philosophical speculations. In 1543 the Polish astronomer Copernicus proposed that the earth orbits the sun, rather than the other way around, but he had no proof of what was to him a mathematical issue. Two generations later the great Italian astronomer Galileo Galilei supplied this proof in the form of telescopic observations of the phases of Venus and the moons of Jupiter. He took up the heliocentric (suncentered) cause, but ran afoul of the authorities for his methods. The truth was out, however, and by the mid-1600s it was universally accepted (in Europe, at least) that the earth orbits the sun. Long before anyone knew whether it was the sun or the earth that moved, astronomers plotted the apparent path of the sun in the sky, relative to the background stars. This path is known as the ecliptic, and it can be displayed in Starry Night. Astronomers also noticed how the rise and set points of the sun on the horizon and its noon-time elevation varied with the changing seasons. They even determined the length of the year - sometimes with surprising accuracy. The Zodiac The suns path among the stars has been considered special since it was first identified. The sun moves through certain constellations, and even in the earliest times these constellations were accorded extra importance. The moon and planets pass through the Section 1 - Astronomy Basics 36 same constellations (plus several others - see The Extended Constellations of the Zodiac in chapter 3.3), and this also contributed to their mystique. Although you cannot see the suns path through the stars when you stand outside, Starry Night can show it to you. The moon stays close to the suns path and provides a simple way to divide it into segments. The moon travels around the sky a little over 12 times while the sun travels around it once (this is the long way of saying there are 12 months in one year; see chapter 4.1). Rounding this to the convenient whole number 12 suggests that the suns (and moons) path be divided into 12 segments, each of an equal length (30). Doing this links the motions of the sun and moon at least symbolically. Along the suns path are prominent groups of stars, like Scorpius and Gemini, and areas devoid of bright stars, like Aquarius and Cancer. The sun passes through 13 of the 88 constellations mentioned earlier in its yearly journey through the stars. 12 of these 13 constellations are the classical constellations of the zodiac. They were all named by 600 BC, but most are far older. Scorpius, for example, has been seen as a scorpion for at least 6,000 years, which is long before the concept of the zodiac as a complete circle was worked out. Most people associate the zodiac with astrology. The moon and planets all stay close to the ecliptic line. 37 Motions of the Earth The 13th Constellation of the Zodiac The constellation boundaries were arbitrary until recently, and each astronomer (and astrologer) was free to place the boundary lines where he saw fit. This caused endless confusion until 1930, when the constellation boundaries were fixed - among astronomers at least - by international agreement. (See The Official Cygnus in chapter 5.2) One effect of this tidying-up was to draw the huge and ancient constellation Ophiuchus so that it intersected the ecliptic. The sun is actually within the boundaries of Ophiuchus from approximately November 30 to December 17 each year (See chapter 3.3 to find your astronomical birth constellation). In contrast, the sun is within Scorpius for only one week. Annual Changes in the Stars The suns apparent motion against the background of stars also causes seasonal changes to the constellations that we see at night. Each day the sun is nearly 1 to the east, relative to the stars, of where it was the day before. If we think of the sun as staying relatively still (and - after all - our timekeeping methods are based on the position of the sun, rather than the stars), we can think of the stars as moving westward 1 per day relative to the sun. Stars rise four minutes earlier each day, or 1/2 hour earlier each week, or 2 hours earlier each month, or 24 hours earlier each year. This is another way of saying that the cycle has been completed and the stars rise at the same time again after one year has passed. If a star rises at 2 a.m. on one date, it will rise at midnight one month Section 1 - Astronomy Basics 38 later, at 10 p.m. another month later, and at 8 p.m. yet another month later (some stars near the North Star are exceptions to this rule, for they are circumpolar, meaning that they do not rise and set, but remain above the horizon all day and night). This is a handy rule of thumb to remember when you are planning which stars and constellations to observe. If you have to stay up too late to see it now, wait a few months and it will be conveniently placed in the evening sky. This rule of thumb does not work for the moon or for Mercury, Venus, and Mars, as they have their own motion against the stars. It is relatively accurate as a rule of thumb for Jupiter and Saturn (and the outermost telescopic planets) and for most asteroids, because they orbit the sun very slowly and thus appear to share the same motion as the stars. Of course, the stars also set four minutes earlier each day, 1/2 hour earlier each week, and 2 hours earlier each month. If Saturn or Orion sets at 8 p.m. this month, it will set at 6 p.m. next month - and you wont see it. What the sun giveth, the sun taketh away. Motions of the Earth Starry Night Exercise This exercise will help you understand how the rotation of the Earth and the revolution of the Earth around the Sun alter the appearance of the sky. You should open Starry Night on your computer before beginning. The Earths rotation on its axis is responsible for the rapid changes in the position of the Sun, planets and stars in the sky. This example shows why some stars are circumpolar. 1) Open a new window by choosing File | New. 2) Select Sky | Daylight to turn sunlight off. 3) Choose Go | Viewing Location to bring up the Viewing Location window. Click the Lookup button and select Toronto from the list of locations. Click OK. Make sure that the box DST is not checked and then press the Set Location button. 39 Motions of the Earth 4) Set the time to 10:00 PM and the date to October 6, 1999. Scroll around using the mouse so that you are looking north instead of south. 5) Open the Find dialog and type Polaris to center on the North Star. The Big Dipper is to the bottom left of Polaris, with its bowl almost parallel with the horizon. Note how the two pointer stars on the end of the Big Dippers bowl form a straight line which points to Polaris (make sure that your Starry Night window is full size so that you can see the pointer stars and Polaris onscreen at the same time). 6) Set the time step in the time controls to 3 minutes, if it is not already set to this value. Now press the Forward button to start time running. You can see that the stars appear to circle counter-clockwise around Polaris. Watch how the Big Dipper changes its orientation relative to the North Star. 7) Notice how the speed at which a star moves is determined by its angular separation from Polaris. The farther from Polaris a star is, the more rapidly it moves in the sky. Stop the time by pressing the Stop button in the time controls. 8) The angular distance of an object from Polaris also determines how long a star stays above the horizon. Use the angular separation tool to draw a line from Polaris pointing straight down to the horizon (scroll down a little if the horizon is not visible). How far is Polaris from the horizon? You should find that it is about 43 degrees, which is also the latitude of Toronto. 9) From Toronto, any star which has an angular separation from Polaris of less than 43 degrees is circumpolar, meaning it is always above the horizon. Use the angular separation tool to measure the angular separation between each of the stars in the Big Dipper and the North Star (if the Big Dipper is not visible, run time forward until it becomes visible again). You will find that they are all less than 43 degrees, meaning that the Big Dipper is circumpolar as seen from Toronto. This example shows you how the position of stars changes with the seasons. We say that Orion is a winter constellation in the Northern Hemisphere. Why is that? 1) Set the date to Dec. 21, 1999, and the time to 8:00 PM. 2) Open the Find dialog and type Betelgeuse (watch the spelling!). Betelgeuse is the bright star in Orions shoulder (note the 3 bright stars to its right which make up the belt). Betelgeuse has recently risen above the horizon. Section 1 - Astronomy Basics 40 3) Right-click (click and hold on a Macintosh) on Betelgeuse to bring up a contextual menu. Select Centre\Lock from this menu 4) Now double-click on Betelegeuse to open its Info Window. Write down its rise and set times and then close this window. 5) Change the time step from 3 minutes to 1 day (choose the step marked day, not the one marked day (sidereal). A sidereal day is a different unit of time which is explained in chapter 3.1). 6) Use the Single Step Forward button to move time ahead by one day. Betelgeuse is now farther up and to the right relative to the horizon. Double-click on it again to find its new rise and set times. You should find that they are both about 4 minutes earlier than the previous times. As you learned earlier, this is due to the Earths revolution about the Sun. 7) Click the Forward (or Play) button to watch Betelgeuses location in the sky change continuously. Stop time just before it falls beneath the horizon. What is the date? You should find that it is near the beginning of June. 8) Click on the small outline of the Sun shown beside the time in the Control Panel. This should color the Sun yellow, meaning that Daylight Savings Time has been turned on. 9) Double-click on Betelgeuse to find its new rise and set times. You should notice that its rise and set times mean that it is now above the horizon only during the day, so it will be invisible to us (remember that we turned off daylight for this example). But if you compare the rise and set times with those you found for Dec.21, you will notice that the total time which the star is above the horizon is exactly the same: 13 hours and 2 minutes! So a star or constellation is classed as winter or summer not because it is above the horizon longer at those times, but because it is above the horizon at night, when we can see it. 10) Now change the time to Dec. 21, 2000. Click on the small outline of the Sun to turn off Daylight Savings Time. Double-click on Betelgeuse to find its rise and set times. You should find that they are the same as a year ago, as the Earth has returned to its previous place in the solar system relative to the sun. 41 Motions of the Earth To close this exercise, we will do something you can never do in real life: watch the Sun move through the constellations of the Zodiac. Remember that this is caused by the Earths revolution around the Sun. 1) Set the time to 12:00 PM. We want to follow the Sun over the course of a year, so we have to choose a time of day when it will always be above the horizon. 2) Scroll around the sky to find the Sun, and then right-click (click and hold on the Mac) on it to bring up a contextual menu. Choose Centre/ Lock from the menu. 3) Turn on the Ecliptic line by choosing Guides | The Ecliptic (Guides | Ecliptic Guides | The Ecliptic in Starry Night Pro). 4) Now turn on the Zodiac constellations by selecting Constellations | Zodiac ( in Starry Night Pro, you need to choose Guides | Constellations | Zodiac Only and then choose Guides | Constellations | Stick Figures). Turn on the constellation labels and boundaries as well. The Sun should be in Sagittarius. 5) Make sure that the time step is still set to 1 day. Start time running by pressing the Forward (or Play) button in the time controls. If you follow the motion for a year, you can see how the Sun moves through all the constellations of the Zodiac, as well as moving through a part of the sky not in the classical Zodiac. 6) Stop time when the Sun is outside the boundaries of the Zodiac. Place the cursor over the Sun. The screen will display the name of the Sun and the constellation it is in, which you will see is Ophiuchus. This concludes the exercise for this chapter. Blank Page 1.3 THE SOLAR SYSTEM Motion of the Moon Is the moon out tonight? Very likely - it is half the nights. If not, it is probably out during the daytime. Many people are surprised to see the moon during the day, but it is visible in the day for two weeks each month. Only for about three days each month when the moon is new or nearly new can it not be seen at some time of the day or night. The moon orbits the earth in counterclockwise direction as seen from above the earths north pole. We see it move night by night across the sky from west to east. Each evening it is about 1/30 of 360, or 12, east of where it was the night before. It takes about 30 days to move through the constellations of the zodiac, and it spends about 2-1/2 days in each, on the average, before moving on to the next. Section 1 - Astronomy Basics 44 In addition to moving around the sky, the moon changes its shape. Planets move too, but the moon and rare bright comets are the only naked-eye objects in the sky that change their shape day by day. In the case of the moon, the change is quite predictable. The chief principle is that the illumination that strikes the moon comes only from the sun, and the sun lights up the side of the moon that faces it. The sun lights up exactly one half of the moon at any moment - but that is true for any ball lit by the sun. Hold a tennis ball in sunlight, and no matter how you turn or position it, the half facing the sun is the half that is lit. As the moon orbits the earth, the side facing the sun remains lit. When the moon is new, the side facing the sun faces away from the earth, and we see the dark side of the moon. This is not the same as the back side, which is the side facing away from the earth; the moons back side and dark side coincide only at full moon. The diagram above shows the Moon at eight positions on its orbit, along with a picture of what the Moon looks like at each position as seen from Earth. A B C D E F G H A B C E G H D F 45 The Solar System As the moon moves out of alignment with the sun, it appears to the east of the sun in our sky, and it sets after the sun. We now see most of the dark side, as before, but we begin to see a thin crescent of the lit side - we see a crescent moon in the evening sky. One week after it is new, the moon has traveled one-quarter of the way around the earth and around the sky - and now it looks like a halfmoon. We see half of the lit side and half of the dark. After another week the moon has moved another quarter of the way around the earth and is now opposite the sun, and we call it a full moon. We are between the moon and sun and we see the moons lit side. Now - and only now - is the moons back side the same as its dark side. After another week, the moon has moved three-quarters of the way around the earth and is in a similar position to when it was at firstquarter, but now the left side of the moon is illuminated, rather than the right. It looks like a half moon. During the final week of the lunar month it is a crescent again that grows progressively slimmer. During the period between first and last quarter, the moons phase is called gibbous. It is a waxing gibbous between first-quarter and full and a waning gibbous between full and third quarter (waxing and waning are from Old English and mean increasing and decreasing respectively). From new to first quarter it is a waxing crescent, and between third quarter and new a waning crescent. Eclipses A solar eclipse happens when the moon moves in front of the sun and blocks the suns light. The eclipse can be either partial (the moon does not completely cover the sun and blocks only part of it) or total (the moon completely covers the sun). A partial eclipse can be annular if the moon moves directly in front of the sun but is not large enough to completely cover it, leaving a ring of sunlight Section 1 - Astronomy Basics 46 (annular comes from the Latin word for ring) shining around the edge of the moon. Annular eclipses exist because the orbit of the moon around the earth is an ellipse, not a circle. This means that the moon is not always the same distance from earth. When it is farther from earth than normal, it does not appear as large in the sky. Therefore, it cannot completely cover the sun and we see an annular eclipse. Only during a total eclipse does the sky grow dark and the suns dazzling corona burst forth in radiant glory. By an accident of the earths history, our sun is about 400 times larger than our moon, but also 400 times farther away. This means that both objects appear to be almost exactly the same size. For this reason, total eclipses Observing Solar Eclipses The suns surface is too bright to look at without risking serious eye damage, and this remains true even when most of the surface is covered by the moon. Always use proper precautions when observing the sun, both when watching an eclipse and when looking for sunspots. Use an approved dark or mirrored filter placed in front of your eye or in front of your telescopes main lens or mirror (do not use a filter that screws into your eyepiece) or project the suns image onto a white surface. Do not use home-made filters which may block the suns visible light but allow harmful amounts of infrared or ultraviolet light to pass through. You can purchase solar filters from a variety of sources. Only during the brief totality phase of a total eclipse it is OK to look at the sun without filters - and you should, to enjoy the spectacle! 47 The Solar System are very brief, because the moon and sun have to be lined up almost exactly, and the motion of the moon prevents a total eclipse from lasting more than a few minutes. During each eclipse, a similar sequence of events happens. Open the file SunEclipse1 to follow the course of a partial eclipse as seen from San Diego, California, in October 2014. The moon appears as a pale disk near the sun. The sun will appear to stand still while the moon approaches, eclipses it, and then moves on. First contact is the moment when the moons edge first touches the sun and the eclipse begins - in this eclipse it is at 1:15 p.m. Second contact is when the moon leaves the sun and the eclipse ends; in this case it is at 3:42 p.m. As seen from San Diego, the moon never covers more than a third of the suns face. For total and annular eclipses there are four stages. Open the file SunEclipse2 (an eclipse widely seen from Mexico in 1991). First contact is at 10:43. Second contact now becomes the moment when the moon first completely covers the sun; it is at 12:10 p.m. The eclipse is total between second and third contacts, and totality lasts only a few minutes (about 7 minutes for this eclipse). The eclipse ends at fourth contact (1:39 p.m.). You may wish to view the moment of totality again, in slower motion. Stop time and change your time step to 1 second. Then change the time to about 12:08 p.m. before starting time running forward again. Annular eclipses follow a similar sequence except that between second and third contacts the moon is silhouetted against the sun, which appears as an unbroken ring for a few minutes. Open the file SunEclipse3 to view an annular eclipse as seen from near Oklahoma City in the spring of 1994. Notice how the sky does not darken completely, even though the sun is almost completely covered Section 1 - Astronomy Basics 48 by the moon at the eclipses peak. The precise times and circumstances of a solar eclipse depend on your observing location. You stand in the shadow of the moon during a solar eclipse, and the central part of the moons shadow (the umbra) is no more than 269 km (167 miles) wide when it reaches the earth. If you are within it, the eclipse is total, but if you are not, it is partial or you miss it entirely. The moon is moving and so is its shadow. The moons shadow sweeps over the earth at about 1600 km (1,000 miles) per hour, racing from west to east. Where the shadow first strikes the edge of the earth, observers see the sun (which is directly behind the moon) low in the east; it is a sunrise eclipse. People at the center of the shadow experience the eclipse at noon, and people at the eastern edge of the earth see it at sunset. Although the path of totality (or A Solar Eclipse From the Moon People on the moon during a total solar eclipse would see nothing unusual happen to the sun, but they would see the shadow of the moon race across the earth. It would resemble a lunar eclipse as seen from earth, but the moons shadow is far smaller than the earths shadow and it appears as a dark splotch with a black center that moves across the surface of the earth. Open the file SunEclipse4 to see the famous July 1991 eclipse from the moon. Everywhere within the area of the large shadow experiences a partial eclipse while the tiny black dot in the middle of the shadow marks the path of totality. Zoom in if you want a closer look at the area of totality. 49 The Solar System annularity) is narrow and comparatively few people see a total or annular eclipse, the outer part of the moons shadow is wide and much of the earths population sees a partial eclipse at the same time. The solar eclipse has a less-spectacular counterpart in the lunar eclipse, caused by the earth passing between the sun and the moon. Lunar eclipses are described in more detail in section 4.1. Planets While the stars appear in the same configuration relative to each other every night, the planets do not. Much like the sun, they move slowly through the sky from constellation to constellation. Some planets move much faster than others. All the planets move through the same constellations - more or less - as the sun. This is because the planets all move around the sun in nearly the same plane, orbiting above the suns equator. From earth, we see this plane edgeon and it appears to us as a line called the ecliptic. The sun always remains on this line and the planets never stray far from it. Although the sun, moon, and planets move across the sky and it appears that we are at the center of all we see, we know now that this is not the case. The sun is at the center of our solar system, and the planets (including the Earth) circle around it counterclockwise as seen from above the Suns north pole. The planets all move in the same direction, but not at the same speeds. The closer a planet is to the sun, the faster it moves because it feels the pull of the suns gravity more strongly. Planets near the sun also have shorter paths and complete their years more quickly; Mercury orbits the sun in only 88 earth days but Pluto takes 248 earth years. Each planet travels at a nearly uniform speed on a nearly circular orbit. (Mercury and Pluto are the only noticeable exceptions to these last two, but they Section 1 - Astronomy Basics 50 follow the general rule of distance by moving fastest when closest to the sun.) Perhaps the most profound observation - and one that would have astounded all ancient people - is that the earth is one of the planets and moves with and among the others. We view the sky from a platform that is itself in motion. The Solar System Starry Night Exercise After working through this exercise, you will know how to use Starry Night to learn more about the motions of the Moon and planets. We begin by watching the motion of the Moon. 1) Open a new window by choosing File | New. 2) Turn off daylight by selecting Sky | Daylight. 3) If it is not already open, open the Planet List by selecting Window | Planet List (Window | Planets in Starry Night Pro). 4) Click on the name of the Moon in the Planet List, and then click the Lock icon. This should center the Moon. If a window comes up saying the Moon is beneath the horizon, click the Reset Time button (the Hide button in Starry Night Pro) in this window. 5) Two fields in the Info Window can tell you about the Moons age. Double-click on the Moon to open the Info Window. Disc Illumination (just called Illumination in Starry Night Pro) tells you what percentage of the moons face is illuminated, ranging from 0% at new moon to 100% at full moon. Age tells you how long it has been since the Moon was last new, and also gives the phase name. The moon rise and set times are also shown (On Starry Night Pro, you need to click the Position tab to see this information). Close the Info Window. 6) Turn on the Zodiac constellations and the Ecliptic line.. 7) Change the timestep to 1 day. 51 The Solar System 8) Press the Single Step Forward button to watch how much the Moon moves in a day. Keep pressing the Single Step Forward button to watch the phases change (if the moon falls beneath the horizon, continue clicking the Single Step Forward button until it reappears). Doubleclick on the Moon at any time to find out its age and phase. Note how the moon stays close to the ecliptic, always remaining within about 5 degrees. We will move on to study the motion of the planets. 1) Select Labels | Planets/Sun (Sky | Labels | Planets | Sun in Starry Night Pro) to turn on labels for all the planets. Scroll around the sky and notice how all planets above the horizon are near the line of the Ecliptic. 2) In the Planet List, click on Jupiters name and then click the Lock icon. If Jupiter is beneath the horizon, click the Reset Time button (Hide button in Starry Night Pro) in the window which comes up. 3) Start time flowing by pressing the Forward (or Play) button (if Jupiter falls beneath the horizon, just continue to let time flow until Jupiter comes back above the horizon). You will see that Jupiter also moves through the Zodiac constellations, but much slower than the Sun did. To see why this is so, we need to change our viewpoint to see the motion of the planets from a new perspective. 4) Stop the time. 5) Choose Go | Viewing Location. Select Sun from the drop-box near the top of the Viewing Location window which appears. Then change your latitude to 90 degrees N and press Set Location. You are now on the north pole of the Sun! 6) Click on the Suns name in the Planet List and then click the lock icon. You are now looking back down on the Sun. 7) Click the arrow in the Elevation box. This box is right near the rocketship buttons and should read 3m intially. Select 1 ly from the list of elevations which appear in the menu. You are now 1 light year above the Sun. 8) Choose Go | Viewing Location again. In the window which comes up, check the box marked Hover. 9) In the Planet List, turn on the orbits of every planet. You turn on a planets orbit by clicking in the column along the right side of the Planet List beside each planets name (in Starry Night Pro, you click on the leftmost of the 3 columns). You wont be able to see the orbits yet, because you are so far above the solar system. Section 1 - Astronomy Basics 52 10) To make sure all the labels will show up, select Labels | Label Settings (in Starry Night Pro, choose Sky | Labels | Label Options). This brings up the Label Options window. On the right side of this window, click on the name Planets (dont click on the checkbox beside the name- it should already be checked). On the right side of the Label Options window is a checkbox marked Label Even When Very Dim. Make sure this box is checked. 11) Now zoom in on the sun by using the zoom buttons. Keep zooming in until the orbit of Saturn almost fills the screen. 12) Change the timestep to 10 days. Press the Forward button to start time running. You can see how quickly the inner planets move compared with Saturn and Jupiter. The outer planets of Uranus, Neptune and Pluto move even slower. This is why Jupiter moves through the Zodiac so slowly. This concludes the exercise for this chapter. 53 Skywatching Section 2 Observational Advice STARRY NIGHT is a great tool for learning more about astronomy, but it will never replace stepping outside and observing the wonders of nature with your own eyes. This section gives you some help on getting more out of your observations. Chapter 2.1 gives general hints & tips, as well as brief advice on buying binoculars or a telescope. Chapter 2.2 explains the different astronomical co-ordinate systems, which you will use to help you locate objects in the sky. 2.1 SKYWATCHING Hints & Tips Although casual skywatching takes no special training or skills, there are some things to keep in mind that will improve the experience. First, as often as possible, observe away from city lights. Through all of history - until this century - the sky was dark at night even in the worlds major cities, and (ignoring chimney smoke) the stars shone as brightly over New York City as over a rural Kansas town. Our modern cities are bathed in a perpetual twilight from the millions of lights that illuminate the air itself, not to mention any dust, smoke, and smog in the air, and this lets us see only the brighter stars. Astronomers refer to this unwanted stray light as light pollution and some campaign against inefficient lighting. Children who grow up in the city do not know what the stars look like and have no Section 2 - Observational Advice 56 appreciation for the beauty of the sky, and even adults often forget. It is common to hear an audience gasp and even applaud when the stars come out in an urban planetarium show. People are most aware of the poor quality of an urban sky when a bright comet appears and they are told by the news media to go to a dark site to see it properly. Even if a comet can be seen from within a city, it is a pale shadow of what people see from a dark location. Many people conduct their most rewarding stargazing while on vacation. The national parks and national forests are generally exceptionally dark at night, and the sky becomes an attraction once darkness has settled over the trees, meadows, and waterfalls. Take binoculars or a spotting telescope on your next vacation and continue to sightsee after sunset. A different type of light pollution is caused by our moon. Even the darkest locations suffer when the moon is bright. For two weeks each month centered on the date of the full moon, moonlight bathes the sky in its own whitish twilight, reduces contrast, and obscures faint objects. There is nothing one can do about moonlight other than to try to work around it. During the week before full moon, the moon sets before sunrise and the sky is fully dark only before morning twilight begins; during the week after full moon, the sky is dark briefly between the end of evening twilight and moonrise. Another observing consideration is comfort. In most places it is cold in winter, especially at night, and winter stargazing means keeping warm. It is one thing to shovel snow or walk briskly to the store on a cold day; it is quite another to stand motionless and watch Orion in the dark. Your body generates little heat while watching the stars, and most people must dress warmer than anticipated. This is often true even in the summer, when it cools off at night. Padded footwear is important in the winter if you will be standing on cold ground. Many people watch meteor showers from the comfort of a 57 Skywatching sleeping bag on a lawn chair or ground pad. Bugs can be a problem too. Bug spray is an essential accessory for many summer stargazers (but be sure to not get spray on your optics!). Another useful skywatching accessory is a red flashlight. Astronomy specialty shops sell observers flashlights that shine with a very pure red light, but you can adapt a conventional flashlight by fitting a few pieces of red gel purchased at an art supply store over the lens. If you want to run Starry Night outdoors on a laptop during your observing sessions, you may also wish to make a red gel cover to fit over your computer screen (unless you own Starry Night Pro, in which case you can just turn on its night vision mode). Using a red flashlight preserves your night vision. Step outdoors from a brightly lit room and notice how long it takes your eyes to adapt to the dark. After a few minutes your pupils will adjust to the lower light levels, and you will be able to see more stars. Turn on a white flashlight and look at a printed star chart. When you look up at the sky again, you will notice that your enhanced night vision is gone. Once you have regained it, look at the same printed chart with a red flashlight and then look back at the sky to appreciate the improvement. Using red lights to preserve night vision is a familiar trick to submariners and others who may have to suddenly leave a lit room and step outdoors ready to see in the dark. Binoculars and Telescopes Your eyes are a perfectly fine tool for stargazing, and there is much to see with them alone on a dark night. But all sky watchers yearn to see more and fainter objects and to see them better, and eventually they want to buy a telescope or pair of binoculars. At the Griffith Observatory in Los Angeles, we recommend that people on a budget (or those who are very concerned with portability) start Section 2 - Observational Advice 58 with a pair of binoculars. Binoculars are under-appreciated and are wonderful for many kinds of casual stargazing, and even experienced observers enjoy having a pair of good binoculars for widefield views of the Milky Way (and for their portability!). You shouldnt leave home without a pair. Binoculars have the advantages of being relatively inexpensive, highly portable, easy to use, and they give the best views of the Milky Way (and often of bright comets). Plus, you can use them for non-astronomical sightseeing, especially while on vacation. Expense is relative, and in general you get what you pay for. Do not be tempted by $50 binoculars at discount camera stores and drug stores. They use plastic parts (including lenses!), are not well assembled, and should only be reserved for situations where they are at risk (such as boating, or perhaps for use by an accident-prone child). Anticipate spending $150 or more for a decent pair that will give good color-corrected images and that will provide a lifetime of use and enjoyment. A good pair of binoculars can be enjoyed for decades and handed down to the next generation. Astronomers have different requirements from sportsmen, and their choice of binoculars is somewhat different. Adequate light is seldom a problem at the horse races, but it certainly is when peering at the Milky Way, so astronomers want binoculars with large lenses. The lens size is expressed as its diameter in millimeters, and it is the second number that characterizes a pair of binoculars. Astronomers want 50 mm lenses (or larger), and they use smaller binoculars only when size and weight is critical, as when backpacking. Larger binoculars with 70 or 80 mm lenses are great for astronomy, but they are expensive and they must be used on a tripod. The other characteristic of binoculars is the magnification, which appears as the first number. Magnification is not so important, and in any case comes within the range of 7 to 12. 10-power is a popular compromise, although it is hard to hold them steady without a tripod. 59 Skywatching The best all-purpose binoculars for stargazers, then, are 7X50 to 10X50; seven to ten power with a lens diameter of 50 mm. Test before buying. If you wear eyeglasses to correct for distance but have no significant astigmatism, use binoculars without glasses. Check before buying that they will focus at infinity to your sight with your glasses off. You probably wont be able to focus on an object which is far enough away while you are inside a store. Ask to test them outside and focus on a distant tree or building. Some people who normally wear glasses use contact lenses when observing, especially to avoid problems when switching back and forth between the sky, a finder scope, binoculars, the eyepiece, and star charts. One step up from binoculars - and a good starter telescope - is Light Collecting Area A 50mm binocular lens does not sound much larger than a 35mm lens, and a 20 cm mirror does not sound much larger than a 15cm mirror, but in both cases the larger is twice the size of the smaller - if you are comparing the light-collecting area. A large lens or mirror collects more light than a small one and reveals fainter objects, and this is vitally important in astronomy. Compare the surface area, not the diameter. The area of a circle is proportional to the length of the diameter squared (the diameter times itself ), so a small increase in diameter makes a big difference in area. The ratio of the lens area of 50mm and 35mm binoculars is 502/352 = 2500/1225, or two to one; similarly, the ratio of a 20cm to a 15cm telescope mirror is 400/225 or almost two to one. Section 2 - Observational Advice 60 a spotting telescope. These telescopes are easy to use and feature low magnification and a wide field of view. While standard binoculars have a magnification of 12X or less, a spotting telescope will reach 40 power or more, giving great views of the moon and bringing in the moons of Jupiter, the rings of Saturn, and many double stars. Theyre great for watching wildlife as well. Their name comes from their use spotting the hits in target practice, but many amateur astronomers have a spotting telescope for vacations and travel. Prices range from $100 to well over a $1000 for a 70 - 100 mm spotting telescope with eyepieces, tripod, and carrying case or bag. Recommendations for purchasing an astronomical telescope are beyond the scope of this book, but here are a few hints. Amateur telescopes come in three flavors: refractors, which use a lens; reflectors, which use a mirror; and hybrids called Schmidt- Cassegrains. All telescopes also use eyepieces to magnify images. Refractors are easier to make in small sizes and they dominate the market in the lowest price range; they are what you see in toy stores. Reflectors are more complex and are seldom sold for less than $200. Larger refractors with a lens 8 cm (3 inches) in diameter or larger can be superb instruments, but they are more expensive than a reflector of the same size and are starter telescopes only for the rich and famous. Reflecting telescopes with mirrors in the 15 to 20 cm (6- to 8-inch) range are the workhorses of amateur astronomers. Reflecting telescopes with the Dobsonianstyle mounting have become popular recently and are a good buy with prices starting at about $500; they are also relatively easy for hobbyists to build using common workshop tools and following published plans. Later you can work up to (or build) telescopes with apertures of Dobsonian-style mounting 61 Skywatching 14, 20, or more inches! Schmidt-Cassegrain telescopes are compact and portable but more expensive for a given size than simple reflectors. Know where to shop - and where not to shop. Avoid discount camera stores (the kind that are always      !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ going out of business), department stores, and toy stores. Stay away from telescopes promoted on the basis of their magnification. A 464-power! wonder is certainly a case of false advertising at best. To avoid disappointment, patronize telescope stores (which unfortunately are few and far between), the better camera stores, and catalogs. Do your homework before you put your money down. Read the magazines Sky & Telescope and Astronomy (sold at major newsstands). They regularly print review articles on telescopes and other equipment for amateur astronomers, and their advertisers are a great source of free information. Request, read, and compare catalogs. Mail-order telescope sales are big business and make sense for people who live far from cities with proper telescope stores. Find out if there is a local astronomy club, and if so, attend a meeting. The clubs members will be a wonderful source of practical information and recommendations. Clubs are also a source of good used equipment. A local planetarium or science museum can also offer advice. Some have large gift shops and sell telescopes and accessories. Last but not least, there are many excellent Internet resources to help you with your search. The major telescope manufacturers maintain sites with extensive information on their products, and there are also many independent sites which compare the merits of different telescopes. Check the web site www.livesky.com for links to some of the best sites. No matter what type of telescope you choose, it is good to have realistic expectations of what you will see through your new instrument. It wont perform like the Hubble Space Telescope! You wont see the colorful images found in astronomy books and Section 2 - Observational Advice 62 magazines; such photographs are long time-exposures taken with professional telescopes. But you will see the real, genuine thing - and that personal experience is worth a lot. 2.2 CO-ORDINATE SYSTEMS Whether you are looking through your new telescope or just gazing up with the naked eye, youll need to have a good understanding of the different astronomical co-ordinate systems to find many of the more interesting jewels in the night sky. What is north? east? south? west? These directions are astronomical. The skys rotation defines directions on the ground, positions on the earth, and the main astronomical co-ordinate system used to specify positions in the sky. In section One, the Altitude- Azimuth co-ordinate system was described. We saw that any object in the sky could be uniquely identified by its angular height above (or below) the horizon and its compass direction. This is called a Horizon or Local co-ordinate system because an objects coordinates in this system are valid only for one location and only for one time. Astronomers use other more general co-ordinate systems to describe the positions of objects in the sky. Of course, it is the earths rotation that determines directions and co-ordinates, but when the co-ordinate system was set up more Section 2 - Observational Advice 64 than 2100 years ago it was almost universally known that the earth is spherical but stationary while the sky turns overhead. The person who invented the system of using longitude and latitude to specify positions on the earth was the Greek astronomer and mathematician Hipparchus. He rejected the notion that the earth rotates on its axis, but his solution works just the same. The earths rotation determines the position of the earths two poles and equator. The poles are where the earths rotation axis penetrates the earths surface as it continues from the earths center infinitely far into space, and the equator is the line midway between the poles. Lines of longitude, called meridians, run from pole to pole and divide the earth into segments that specify a points distance east of the zero segment, which last century was arbitrarily chosen to be the longitude of the Greenwich Observatory in England (the Prime Meridian). Longitude is measured either in degrees, in which case there are 360 around the earths circumference, or in hours, minutes, and seconds, which represents the actual rotation of the earth. If you live 7 hours west of Greenwich, a star is overhead at your location 7 hours after it is overhead at Greenwich. There are 15 in one hour of longitude, which is the amount the earth rotates in one hour of time. Hours are subdivided into minutes and seconds, and as you learned in chapter 1.1, degrees are subdivided into arcminutes and arcseconds. Longitudes in Starry Night are expressed in degrees east Earths lines of latitude and longitude 65 Co-ordinate Systems or west of Greenwich, while latitude is the angular distance of a location north or south of the earths equator, expressed in degrees. The latitude of the equator is 0, of the north pole 90 N or +90, and of the South Pole 90 S or -90. Positions on earth are expressed as their longitude and latitude, in that order, which is the same as the angular distance west (or east) of the Prime Meridian passing through Greenwich and the angular distance north or south of the equator. There is a corresponding system which astronomers universally use to specify positions on the celestial sphere. It is called the equatorial co-ordinate system. The celestial sphere is a concept, not a real object, but it is convenient to think of the globe of the sky as an invisible crystalline sphere that has the stars attached to it, with the sun, moon, and planets moving along its surface (it is an actual aluminum sphere in a planetarium theater). Just as positions on earth are expressed by: 1) the time it takes the earth to rotate from the zero meridian at Greenwich to the specified location, 2) the angular distance north or south of the earths equator, so celestial positions are expressed in terms of: 1) the time the sky takes to rotate from an arbitrary zero meridian to the specified point, 2) the angular distance north or south of the skys celestial equator. The two values are called right ascension and declination respectively, abbreviated RA and Dec. Values in right ascension are expressed in hours, minutes, and seconds; values in declination are expressed in degrees, arcminutes, and arcseconds. For example, the equatorial co-ordinates of Rigel would normally be expressed in this format: 5h 14m, -8 12. To determine an objects right ascension, we first need to choose a line of 0h RA. Much like the line of 0 longitude on Earth is the Section 2 - Observational Advice 66 Prime Meridian, the line of 0h Right Ascension is known as the Celestial Meridian. This should not be confused with the local meridian, which is an imaginary line in the sky running from north to south through the zenith. Which stars the local meridian passes through depends on your location, whereas the celestial meridian always passes through the same stars and constellations. Defining the celestial meridian is a two-step process: we first define the vernal equinox as one of the two points on the celestial sphere where the celestial equator meets the ecliptic (the vernal equinox is also the moment in time when the sun crosses the celestial equator in March; it is the moment when spring begins in the earths northern hemisphere - see Chapter 3.2). The celestial equator is the projection of the earths equator into space, and the ecliptic is the suns apparent annual path around the sky. Open the file Intersection to see how the vernal equinox is one of the two points where the celestial equator and the ecliptic intersect (the second point is called the autumnal equinox, and it is beneath the horizon). Start time running forward to see that the vernal equinox moves in the sky over the course of a night just like a star. It therefore has a fixed place in the celestial sphere, in the constellation Pisces. Having defined the vernal equinox, the celestial meridian is then defined as the line extending from the North Celestial Pole through the vernal equinox to the South Celestial Pole. Once a meridian has been defined, an objects right ascension is determined by its distance from this meridian. An object with an RA of 12 hours has the same RA as the autumnal equinox, while objects with an RA of 6h or 18h are halfway between the vernal and autumnal equinoxes. Declination is perhaps easier to understand than right ascension, because its reference line is the celestial equator, which is just the projection of Earths equator onto the celestial sphere. It 67 Co-ordinate Systems is therefore an exact counterpart to latitude on Earth. We measure declination in degrees north or south of the celestial equator, which itself has 0 declination. The declination of the North Celestial Pole is +90 or 90 N and the South Celestial Pole is -90 or 90 S. The equatorial co-ordinates of a star do not change as the hours pass (Actually, this is not precisely true because of proper motion and precession, but these motions are very slow, and do not change the position of a star enough to notice, at least with the unaided eye, during a human lifetime. See chapters 3.3 and 5.2 for more on proper motion and precession, respectively.) Just as the longitude and latitude of Boston do not change as the earth turns, the equatorial co-ordinates of Rigel remain constant night after night and year after year. The equatorial co-ordinates of a planet or comet do change from night to night as the planet or comet moves across the sky relative to the stars beyond. It is very useful to have a feeling for the equatorial co-ordinate system if you wish to be able to find things in the sky. An invaluable exercise is to stand outside at night and imagine the co-ordinates as if they w e r e chalked on the sky, and to imagine how they turn with the c e l e s t i a l sphere as it celestial equator ecliptic Section 2 - Observational Advice 68 wheels overhead. With Starry Night, you dont have to imagine - just choose Guides | Celestial Grid (Guides | Equatorial | Grid in Starry Night Pro) and let time run forward. The equatorial coordinate system is based on the rotation of the earth and is most useful for specifying the position of an object on the celestial sphere in relation to the earth (to the earths equator and poles). A third system, called the ecliptic co-ordinate system, is more useful when you want to specify the position of an object relative to the sun and to the orientation of the solar system. The counterpart of the equator in the ecliptic co-ordinate system is the ecliptic, which is the earths path around the sun. It is also the suns apparent annual path around the sky - another way of expressing the same concept. Positions in the ecliptic co-ordinate system are specified in ecliptic longitude and ecliptic latitude, both in degrees. The ecliptic itself has an ecliptic latitude of 0, and the ecliptic poles have ecliptic latitudes of +90 and -90. The North Ecliptic Pole - the north pole of the solar system - is in Draco, and the South Ecliptic Pole is in Dorado not far from the Large Magellanic Cloud. Positions along the ecliptic are measured eastward in degrees from an arbitrary starting point, which is again chosen to be the vernal equinox - the point where the ecliptic intersects the celestial equator in Pisces. Ecliptic longitudes are numbered between 0 and 360. Specifying an objects position in ecliptic co-ordinates tells you when the sun is closest to it and how far it is from the suns path. The ecliptic position of Antares, for example, is 250, -5; the sun is closest to longitude 250 on December 2 (you have to look up the date - or let Starry Night show you), and at that time Antares is 5 south of the sun. Ecliptic co-ordinates are almost exclusively used for solar system objects. Planetary conjunctions are usually expressed in ecliptic co-ordinates (see the section on conjunctions in Chapter 4.2). When Apollo astronauts left the earths vicinity to 69 Co-ordinate Systems travel to the moon, they left the earths co-ordinate system behind and used the ecliptic system to navigate. Special ecliptic-based star charts were prepared for their voyages. A fourth co-ordinate system is of specialized use to people interested in the Milky Way. It is the galactic co-ordinate system, and it uses the equator and poles of the Milky Way Galaxy as its fundamental plane and polar points. It lets you specify the position of an object in an east-west longitude sense relative to the center of the Milky Way and in a north-south latitude sense relative to the plane of the Milky Way. The Milky Ways center is in Sagittarius (at coordinates 17h 46m, -29 in the equatorial system) and its north and south poles are in Coma Berenices and Sculptor (at co-ordinates 12h 51m, +27 and 0h 51m, -28) respectively. The system lets you specify a stars position as its galactic longitude and galactic latitude. Only Starry Night Pro can display the galactic and ecliptic co-ordinate systems. There are many other less well-known co-ordinate systems, such as those associated with other planets or moons. These coordinate systems are used by solar-system astronomers when calculating planetary spacecraft trajectories and other phenomena associated with particular planets. The most important co-ordinate systems for finding your way around the sky are the Alt-Az (horizon) and equatorial co-ordinate systems. The co-ordinates you will find in a book are usually equatorial, but you need the horizon co-ordinates to actually know where in the sky you should be looking! Starry Night offers an easy way to get the different co-ordinates of any object. Just double-click on the object to bring up an Info Window. There is a drop-down menu which lists the four co-ordinate systems we have discussed (there are actually two different options for equatorial co-ordinates shown. The difference between these two is discussed in Chapter 3.3). When you select one of these options, the appropriate co-ordinates show Section 2 - Observational Advice 70 up beneath this pop-up menu. This makes it easy to convert between different systems. Remember that horizon co-ordinates are timedependent, so make sure to set the time in Starry Night to the time at which you want to observe. Once you have a good understanding of co-ordinate systems, you will spend less time figuring out where to look, leaving you more time for the observations themselves! Section 3 Earths Celestial Cycles AS WE SAW in section 1, there are several sources for the apparent motions of the stars, planets and other objects that we see in the night sky. We can divide these motions into two categories: the actual motion of the objects, and the apparent motion caused in reality by the earths movements. This section of Starry Night Companion looks at the earths movements. The apparent motion due to the earth can be subdivided into three distinct components: rotation, revolution and precession. These sources of motion are all periodic, meaning that their effect is repeated after a certain time. However, the periods for the three motions are radically different: the earth completes one rotation in a day, one revolution of the sun in a year, and one precession cycle on its axis in 26 000 years! Rotation is covered in chapter 3.1, revolution in chapter 3.2, and precession in chapter 3.3. 3.1 ROTATION Whats in a Day? The time it takes the earth to spin once is a day, but how long is that? Twenty-four hours is the fast and simple answer, but it is just one of several that are correct. Relative to what? is the second half of the question. Our lives are regulated by the appearance (and disappearance) of the sun, and the solar day is the fundamental unit of time. The average time from one sunrise to the next is one solar day, and it does equal exactly 24 hours. That is the time it takes the sun, on average, to return to the same position relative to you and your horizon (on the average is an important qualification because the length of the day changes with the seasons; see Chapter 3.2 for seasonal changes in the suns motion). Section 3 - Earths Celestial Cycles 74 We could also think of the time it takes the earth to spin once relative to the stars. Because the sun moves eastward a slight distance relative to the stars each day, during a sidereal day (sidereal means stellar) the earth turns less than once relative to the sun while it turns exactly once relative to the stars. This makes a sidereal day shorter than a solar day by four minutes. The sidereal day is 23 hours 56 minutes and 4 seconds long. Open the file Day and notice that the sun is on the meridian - the imaginary line that runs from due south to straight overhead and on to the north horizon. Daylight is turned off so that we can see the stars as well as the sun. Some stars near the meridian are labelled. Click the Single Step Forward button to advance the time by one sidereal day. The stars have not moved; more precisely, they have made one complete circle around the North Star and returned Day 1 Day 2 B C A 4 minutes A sidereal day is the time it takes the earth to spin once relative to the stars (point A to B). A solar day is 4 minutes longer (point A to C). Sun Earths orbit 75 Rotation to their original position. But notice that the sun is now about 1 to the east of its position the day before and is not yet on the meridian. Notice also that the date has advanced by one day but the time has decreased by 4 minutes. Now change the time step to 4 minutes and press the Single Step Forward button to complete one solar day. The sun is now back in the same place in the sky, but the stars have shifted slightly to the west. This four-minute period is the difference between a solar and a sidereal day - the difference between one rotation of the earth relative to the sun and one rotation relative to the stars. Change the time step back to sidereal days and continue to step forward in time to make the concept clear: a sidereal day is one rotation of the earth relative to the stars, not to the sun. Hours, Minutes, and Seconds We divide each day into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. These are arbitrary divisions, and we could just as well divide the day into 10 hours of 100 minutes each, as was done following the French Revolution (the French decimal day was unpopular and did not last). Our hours come from ancient Egypt, where the day was divided into 10 hours of equal length with one more each for morning and evening twilight. The night was divided into 12 equal divisions for symmetry, giving a total of 24 hours for the complete cycle. The custom of dividing periods of time into units of 60 dates to the Sumerians, who liked to use numbers that were evenly divisible by many other numbers; 60 is evenly divisible by 30, 20, 15, 12, 6, 5, 4, 3, and 2. Section 3 - Earths Celestial Cycles 76 Time Zones Early last century, most large American cities maintained an observatory, a major function of which was to determine the local time. The time in Detroit was different than the time in Toledo, and in fact it was different in every city across the land. The custom of using a unique local time in each community became more confusing as the speed of communication improved, and with the advent of rapid rail travel it became intolerable. It was impractical to maintain railroad timetables that had to allow for not only the speed of the train but also for local times at each station, and Canadian railroad magnate Sir Sanford Fleming lobbied successfully for standard time zones. In 1884 the earths globe was divided into 24 meridians, each about 15 of longitude apart, starting with the zero (or prime) meridian which passes through London, England. Each time zone is one hour wide. All areas within a time zone have the same solar time, and it is one hour later in the time zone to the east. The time zone boundary lines are not perfectly straight, but generally follow political and natural features for the convenience of people living within them. This means that it is not possible to determine your time zone from your longitude alone (as an extreme example, China spans about 60 of longitude, but the entire country shares the same time zone). Starry Night automatically adjusts the time zone if you select a city from the list in the Viewing Location window. If your city is not listed and you have to enter your viewing co-ordinates directly, you will also need to specify the time zone. You can find time zone information from http://www.starrynight.com. Having standard time zones is a great convenience, but there are astronomical compromises. It may be the same moment by the clock at all locations within a time zone - but the sun and stars are 77 Rotation not at the same place in the sky for all those locations. If the sun sets at 7:01 p.m. for a city at the eastern end of a time zone, it will still be well above the horizon as seen from a city near the western edge of the same time zone, where it will not set until almost 8:01 p.m. Likewise, moonrise and moonset times are not the same for all locations within a time zone. Starry Night gives you customized sunrise, sunset, moonrise, and moonset times for your location, which may differ from times published in local newspapers. To see how rise and set times differ between localities, use Starry Night to find sunrise and sunset times from your home location. Remember that this information is contained in the suns Info Window, which you can show by double-clicking on the sun. Now change your viewing location to another city in the same time zone [select a city by clicking the Lookup button from the Viewing Location menu under the Go menu] and get the new rise and set times to see the difference. Notice that moving north or south has much less effect on the rise and set times than moving an equivalent distance east or west. A further twist to time zones came with the introduction of Daylight Saving Time during World War I. Originally called War Time, the purpose of moving sunset back one hour was to reduce fuel consumption for lighting and increase production in poorly lit factories during the late evening. It was dropped at the end of the war, and then reintroduced during World War II. It was dropped in 1945, and reintroduced most recently in 1967. It presently begins on the first Sunday in April and ends on the last Sunday in October in most of the United States and Canada; in Europe (where it is called Summer Time) it runs from the last Sunday in March until the last Sunday in September. Daylight time is great for kids who like to play outside late at night, but it delays stargazing by the same hour. Section 3 - Earths Celestial Cycles 78 If you have your computer clock set up correctly, Starry Night will detect whether or not daylight time is in effect for your home location on the current date. The small sun icon shown to the left of the time tells you if daylight time is on; when the icon of the sun is bright yellow, daylight time is on, and when the icon is dimmed, daylight time is off. Just click on this icon to turn daylight time on or off. At any given moment - right now, for example - it is 24 different times around the earth - one time for each time zone (a messy complication is that some jurisdictions have chosen to set their clocks using fractions of a time zone and are one-half or even one-quarter of an hour off standard time). This is a problem if you want to publish the time of an event in a table that is useful for the entire world. Astronomers find it effective to express events in Universal Time, abbreviated UT, which is the local time at the Greenwich Observatory in London, England. You need only know the difference between your clock time and Greenwich time to apply corrections (for example, Eastern Standard Time is five hours earlier than Greenwich). Converting local times back to Universal Time when communicating with other observers around the world is a convenience to people who then need only to apply the familiar conversion of Universal Time to their own time. If you use Starry Night to view the sky from somewhere other than the Earths surface, the time will automatically be shown as Universal Time. Astronomers often need to know the interval between two times separated by days, months, or years - for example, the exact interval between two observations of a variable star made months apart. Our calendars many irregularities make it laborious to find, for example, the interval between 1:17 a.m. March 15, 1988 PST and 4:06 a.m. September 12, 1999 EDT. The Julian Day system lets you do this easily. This system assigns consecutive numbers to days 79 Rotation starting at an arbitrary day zero in 4713 BC. To further simplify calculations, the Julian Day system uses decimal days rather than hours and minutes. Convert each of the two dates to a Julian Day, and then subtract. In the example above, the first date converts to Julian Day number 2447235.88681 and the second to Julian Day number 2451433.83750, and subtract; the answer is 4197.95069 days. To find the Julian Day equivalent of any date and time using Starry Night, first change the time to the time you are interested in. In Starry Night Backyard, click on any object in the Planets List and then click the Edit button in the same window. This brings up the Orbit Editor window, which displays the Julian Day in the upper left corner. In Starry Night Pro, just choose Guides | Onscreen Info Settings from the main menu and then click the Julian Day option. The Nightly Rotation of the Sky The sky turns overhead day and night, rotating as if it were a giant crystalline sphere (which it was thought to be during Medieval times) with the sun, moon, planets and stars attached to it. It rotates as a single unit at the rate of 1 r.p.d. (rotation per day). Revolution vs. Rotation Two words that are confused endlessly are rotation and revolution revolution. Objects rotate on their axes and revolve around another object. The earth rotates once a day and revolves around the sun once a year. The word rotate has an a for axis, which is what an object rotates on. Section 3 - Earths Celestial Cycles 80 As you saw in the exercise for chapter 1.2, the North Star, Polaris, stays motionless in the sky. It is the pivot around which the rest of the stars turn. Or at least it appears to be. Open the file NCP to watch the rotation of the stars. Now zoom in using the Magnify button until your field of view is about 10. You can now see that Polaris is not really motionless, and it makes a circle like any other star. The true pivot point is the North Celestial Pole, which is the point in the sky directly above Earths North Pole. Turn on the display of the North Celestial Pole by selecting Guides | Celestial Poles (Guides | Equatorial | Celestial Poles in Starry Night Pro). Polaris is not exactly at the skys pole, but is 3/4 from it - pretty close! This means that it makes a circle around the North Celestial Pole, 1-1/2 in diameter, and the North Star is never more than 3/4 from true North - too small a discrepancy to make a difference for most navigators. When you have finished watching the motion of Polaris, close the file NCP. Stars near the North Star make small circles around it, completing one circle in 24 hours. Stars farther from the North Star make larger circles, but still one circle per 24 hours. Stars far enough from the North Star set below the northwest horizon, disappear briefly below the northern horizon, and then rise again in the northeast. As you learned in the exercise for chapter 1.2, the far northern stars that do not set are called circumpolar (in the Southern hemisphere, it is the far southern stars that are circumpolar). They were considered immortal in ancient Egypt and they were associated Long time-exposure photograph centered on the North Star. 81 Rotation with the pharaoh - who also was immortal, and who ascended to and lived among the far northern stars once his life on earth was finished. Although we do not think of them as sacred today, the circumpolar stars have the distinction of not setting and thus are theoretically visible through the year (theoretically because they may come too close to the northern horizon to see in practice). The Little Dipper, Cassiopeia, Cepheus, and most of Draco are circumpolar as seen from Canada and most of the United States and Europe. How a star appears to move across the sky as the sky rotates depends on its position on the celestial sphere. If you live at midnorthern latitudes (anywhere within the United States and Canada below the Arctic Circle), a star (or planet, or the sun, or moon) that rises due east reaches its highest point when it is due south (when it is on the meridian), and it sets due west. Stars that rise south of due east do not reach as high a point on the meridian, and they set correspondingly south of due west. Stars that rise far to the south of east make short arcs across the southern sky, are not very high when on the meridian, and set shortly after rising in the southwest. You can easily visualize that there is a portion of the sky that lies farther south than the part you can see and that remains below the southern horizon. Now we will see how a change in your latitude changes the rotation of the sky. We will begin at a latitude of 15 N by opening the setting 15N. Note how low Polaris is in the sky. Find the stars Vega and Eltanin on the left side of the screen. Now press the Forward button in the time controls to start time running. Both Vega and Eltanin fall beneath the horizon before reappearing. The familiar W made by Cassiopeias brightest stars is circumpolar to most observers in the Northern hemisphere. Section 3 - Earths Celestial Cycles 82 Open the file 45N to see how the sky looks from a latitude of 45 N. The North Star is farther above the horizon. Again find the stars Vega and Eltanin on the left side of the screen and start time running. At this latitude, Eltanin is circumpolar, while Vega still falls below the horizon briefly before reappearing. Now open the file 60N to view the sky from a latitude of 60 N. The North Star is still higher. Again find Vega and Eltanin and start time running forward. From this latitude, you can see that both Vega and Eltanin stay above the horizon at all times. It is a rule that as you move farther from the Equator, the number of stars that are circumpolar increases. At the same time, a proportionately greater area of the southern sky remains invisible below the southern horizon, and stars you might have once seen near your southern horizon no longer rise into view. These ideas are carried to their logical extreme at the North Pole. Open the file 90N to view the sky from the North Pole. From here, the entire sky is circumpolar. Start time running to see that no stars rise and no stars set; they all remain visible all the time, and each star maintains a constant altitude above the horizon. The North Star cannot be seen. This is because it is directly overhead. Remember that the altitude of the North Star is equal to your latitude (as you learned in the exercise for chapter 1.1), which is now 90. If you scroll up in the sky, you can see the North Pole almost exactly at the zenith. Scroll around to the south to find Saturn and Jupiter. If you follow their motion, you can see that they (like all the planets, as well as the moon and sun) also circle endlessly at a constant elevation, neither rising or setting. In chapter 4.2, we will see how and why these bodies do rise and set as seen from the pole; they do rise and set, but not because of the earths rotation. The flip side of being at a location where all stars are circumpolar is that many stars and constellations which are promi- 83 Rotation nent from mid-Northern latitudes cannot be seen at all from the North Pole. Choose Edit | Find (Selection | Find in Starry Night Pro) and type Rigel. A window will come up telling you that Rigel is beneath the horizon. You can run time forward and wait as long as you want, but this bright star in Orion will never come above the horizon. From any location, half of the sky is visible at any given moment. However, the closer you are to the Equator, the greater the number of stars that rise and set, meaning that you can see more different stars, but they are above the horizon for a shorter period of time. Now move to the equator by opening the file Equator. You cant see the North Star although it is directly in front of you, because it is right on the horizon. Its altitude is equal to your latitude, which is now 0. If you start time running forward, you can guess its location by seeing how the stars still pivot around a point. Now scroll around so that you are facing south. The stars in this half of the sky pivot around a point on the horizon that is due south. This is the South Celestial Pole. The way the sky turns in the north is symmetrical with the way in turns in the south. Stars that rise due east pass directly overhead and set due west. From this location, all stars rise and set; no stars are circumpolar. Along the Equator (and nowhere else on Earth), every part of the celestial sphere is visible at some point in time. Now we will move south. Open the file 30S to view the sky from a latitude of 30 South. A large area of the sky that includes the Southern Cross is circumpolar. Notice that there is no bright South Star (see the section on Crux in Chapter 5.3) and the sky rotates around an empty, starless point (the few stars near the South Celestial Pole are quite faint, and can only barely be seen in perfect conditions, making them almost impossible to use as navigational aids). Scroll around so that you are facing north, and you will see Section 3 - Earths Celestial Cycles 84 familiar northern constellations in unfamiliar positions (unless, of course, you live in the southern hemisphere); some are upside down and others are on their side. The sun still rises in the east (the earth still rotates so that the sky turns from east to west) - but the sun moves across the northern sky, not the southern as we see it from North America. Finally, travel to the South Pole by opening the file 90S. The South Celestial Pole is directly overhead. All visible stars are circumpolar, and each star maintains a constant elevation as the sky turns. The sky rotates in a similar manner as from the North Pole, but the stars you see are completely different. Not a single star in this hemisphere of the sky can be seen from the North Pole! If you scroll around, you will see the Southern Cross two-thirds of the way up the sky, Sirius is 17 above the northern horizon, and Orion is upside down with his head below the horizon and his feet in the air. From everywhere on Earth, the sky rotates once a day, but how the stars rise and set, which are circumpolar (and which are visible at all), depends on your latitude. No bright star stays near the South Celestial Pole. 3.2 REVOLUTION The second major motion of the earth is its yearly revolution around the sun. It affects how we see the sun, stars, and planets. The Year Just as the day is a unit of time based on the Earths rotation on its axis, the year is based on the revolution of the Earth around the Sun. The time it takes the earth to orbit the sun once - or the sun to circle around the sky relative to the stars - is one year. In chapter 2.2 we found that there are several definitions for something so simple as the length of the day, and it is the same with the length of the year. The length of the annual cycle depends on which reference point is used. If we measure the time it takes the sun to circle the sky once and return to the same position relative to the distant stars, 365 days 6 hours 9 minutes and 10 seconds will pass. This is the sidereal Section 3 - Earths Celestial Cycles 86 year, sidereal meaning stellar or of the stars in Latin. However, it is not the most useful year for most purposes because it does not allow for precession. Precession (which is described more fully in chapter 3.3) is the slow wobbling of the earths axis in a 25,600-year cycle. Precession causes the vernal equinox - and all other points along the ecliptic - to shift in the same long 25,600-year cycle. If we used sidereal years for our calendars, the seasons would slip through the months and eventually the Northern Hemisphere winter would begin in July. As it is more convenient to keep the seasons constant - so that the vernal equinox always occurs on or near March 21, for example - we measure the year as the length of time it takes the sun to circle the sky relative to the vernal equinox. The vernal equinox, you will recall, is the intersection of the celestial equator and the ecliptic. This point of intersection precesses slowly westward along the ecliptic, and it takes the sun less time to return to the vernal equinox than to the same point relative to the stars. This so-called tropical year, on which our calendar is based, is 365 days 5 hours 48 minutes and 46 seconds long from vernal equinox to vernal equinox. The time it takes the sun to circle the sky once and return to the westwardmoving vernal equinox is a few minutes less than it takes the sun to return to the same position among the stars. Open the file Vernal and notice how the vernal equinox moves relative to the stars. Use the Single Step Forward button in the time controls to step forward through time and watch the vernal equinox slide to the right along the ecliptic relative to the stars. It is important to understand how slow this change is occurring. To notice it at all, you are zoomed in to a very high magnification and advancing time by steps of 365 days. Return to a regular 100 field of view and step forward a few more times. At this scale the movement of the vernal equinox with respect to the stars is imperceptible, which is why the difference in the lengths of the two types of years is only twenty minutes. 87 Revolution Seasonal Changes In the Path of the Sun You learned in the section in chapter 2.2 that the ecliptic is inclined to the celestial equator. These two great circles (circles on a sphere) intersect at both the vernal equinox and the autumnal equinox. The angle these two great circles make with each other at that point is 23-1/2, which is the amount the earth is tilted on its axis. You can also think of it as the angular distance between the North Ecliptic Pole and the North Celestial Pole. The technical term for this tilt is the obliquity of the ecliptic. If the earth sat upright in its orbit (as Mercury does), the two poles would coincide - and so would the ecliptic and the celestial equator. This tilt is not just a curiosity - without it, we would not have seasons! Unless you live on the equator, you know that the changing seasons bring changes in the daily motion of the sun. The coming of winter means that the sun rises later, sets earlier, and follows a lower path across the sky. When winter fades into spring and then The 23.5 degree tilt in the earths axis creates the seasons. At the solstice on June 20, the earths northern hemisphere is at a maximum tilt toward the sun, while the southern hemisphere is at its furthest tilt away from the sun. The situation reverses at the other solstice on Dec. 21. The mid way point, called the Equinox, defines Spring and Autumn. 23.5 N N N N S S S S Winter Summer Winter Summer Spring Spring Autumn Autumn Dec. 21 June 20 Section 3 - Earths Celestial Cycles 88 into summer, these changes are reversed. The cause of these changes is the suns changing position along the ecliptic. Open the file South to watch the sky turn as a night passes. We are facing south with the celestial equator and ecliptic displayed, and have chosen to view from Anchorage, Alaska, because the celestial equator is relatively low in the sky and easy to see. Use the Single Step Forward button in the time controls to step forward through time by 5-minute intervals. Notice that the celestial equator (and all lines of declination) remains in the same position all night long. The celestial equator is an arc that extends from east to west and that has a maximum elevation, in the south, that is the complement of your latitude (90 minus your latitude). Points on the celestial equator move westward as the sky turns, but it is important to note that the celestial equator itself remains in the same position all night long. This is true no matter where you are on earth. In contrast, you can see that the ecliptic wobbles as the sky turns. In the northern hemisphere, the part of the ecliptic that runs through Scorpius and Sagittarius is low in the south when it is in view, while the part that runs through Taurus and Gemini is higher and much nearer to overhead when it is in view. This causes the sun - which stays on the ecliptic - to be high in the sky in summer, when it is in Gemini, and low in winter, when it is in Sagittarius. If you keep stepping forward in time until the sun comes into view, you will see that it is along one of the lower parts of the ecliptic, which is not surprising, because the month is December! To see how the suns elevation changes during a year, open the file SunElevation. The location is set for Iceland so the sun will appear low in the sky, but after running this demonstration you should change to your home location and run the demo again. The sun is locked and will stay centered in your screen. 89 Revolution We begin with the sun on the meridian on June 21, which is the summer solstice - the first day of summer. Double-click on the sun to bring up the Info Window. If you choose local co-ordinates, you will see that the sun has an elevation (or altitude) of 49 as seen from Iceland. This is as high as it will be that day. Now use the Single Step Forward button in the time controls to step through one year at the rate of one solar day per step, watching the sun run eastward along the ecliptic while remaining due south and on the meridian. The suns elevation decreases daily, slowly at first - through July - but at a faster rate as summer ends. On August 1 it is 44 high at local noon. (You will see the sun deviate from the meridian slightly. This deviation is called the equation of time and it is explained in the sidebar below) The rate at which the sun loses altitude is greatest on the autumnal equinox, which is on or very near September 22. This should not surprise you, as you have probably noticed in your own experience that the length of the day (the amount of time that the sun is above the horizon) shrinks very rapidly with the onset of autumn. September 22 is the date when the sun crosses the celestial equator in a southward direction, and it marks the first day of autumn in the Northern Hemisphere. As seen from Iceland, the sun has a meridian elevation of 26. This is 23 less than its elevation on the summer solstice. Following the autumnal equinox, the sun continues to move lower at noon each day, but at a slower rate of change. Each day the sun is a little lower at noon than the day before until, on December 21 or 22, the sun reaches its lowest maximum daily elevation. This is the winter solstice, which comes from Latin words for sunstand because the sun stands still (does not continue to lower altitudes) before reversing itself and increasing its elevation. From Iceland, this minimum noontime elevation on the solstice is a scant 2-1/2! The sun barely rises. Section 3 - Earths Celestial Cycles 90 Equation of Time The sun is not a uniform timekeeper for two reasons. First, the earths orbit is not circular, so the earths speed around the sun is not constant. According to the laws of planetary motion discovered by Kepler almost four centuries ago, a planet travels fastest when it is closest to the sun (because it feels a greater pull of the suns gravity when it is closer to the source of the pull). The earths orbit deviates from a perfect circle by less than 2%, but that is enough to notice. The earth is closest to the sun (at perihelion perihelion) early in January each year, and then it is moving slightly faster than when at its farthest point (at aphelion aphelion) early in July. The earths changing speed around the sun makes the sun appear to change speed as it mo ves on the ecliptic ar ound the sky. Second, the sun travels along the ecliptic rather than the celestial equator. The ecliptic is inclined to the equator but we measure time along the equator. Even if the sun traveled at a uniform rate along the ecliptic, its projected motion on the equator would change during the year. The combination of these two effects means that the sun is not in the same place in the sky at the same time each day. Its altitude (height above the horizon) and azimuth (direction along the horizon) both change over the course of a year. If we plot the suns position in the sky at noon each day over the course of a year, it traces out a figure-8 path known as the analemma. To see this in Starry Night, select Go | Analemma. Clocks (and societies) run best at a uniform rate. Your wristwatch ignores the seasonal variations in the 91 Revolution speed of the sun and displays time as if the sun traveled at a constant speed. This imaginary uniform-speed sun is called the mean sun (mean meaning average). The difference between true (or apparent) solar time and mean solar time is called the equation of time time, and it can be anywhere between -14 and +16 minutes. Your watch keeps mean solar time, but a sundial keeps true solar time. After the winter solstice, the sun reverses its downward trend and stands a little higher at noon each day. By the spring or vernal equinox on March 21, it is back to the same noontime elevation as on the autumn equinox: 48. It reaches its highest noontime elevation at the summer solstice in June, when it stands still again before reversing course and heading southward. Follow the path of the sun over an entire year. The changing elevation of the sun means that the relative lengths of day and night also change with the seasons. The winter solstice is the shortest day of the year in the Northern Hemisphere. Between the winter solstice and the summer solstice, the suns maximum elevation is increasing, so the sun is above the horizon longer, hence the days become steadily longer. On the day midway between the two solstices - the vernal equinox- night and day are both 12 hours in length, a fact which holds true anywhere on Earth. After the summer solstice, the suns maximum elevation begins to decrease, so the days get shorter, and keep getting shorter until the Winter Solstice comes again, becoming the same length as the nights on the autumnal equinox. In the Southern Hemisphere, this pattern is exactly reversed. Section 3 - Earths Celestial Cycles 92 Effects of Latitude The maximum daily elevation of the sun changes with the seasons from every location on earth, but the changes become more pronounced as you move farther from the equator. Two sets of latitude lines on the earth are particularly important, and the suns behavior changes when you cross one of these lines. The two important sets of lines are: 1) the Tropic of Cancer and the Tropic of Capricorn 2) the Arctic and Antarctic Circles. You will learn why these lines are important in the next two sections. The Tropics In our view from Iceland, the sun was never overhead. It reached a maximum noontime elevation of 49 on June 21, but went no higher. If you were south of Iceland, the sun would be higher on the meridian on June 21 (and on every other date). If you were 5 south of Iceland, for example, the sun would be 5 higher in your sky at noon on the same date. If you were just north of Mazatln, Mexico, at a latitude of 23-1/2, the sun would just barely reach the zenith on the summer solstice. This latitude, called the Tropic of Cancer, marks the northernmost limit on the earths surface where the sun can be overhead. If you are north of the Tropic of Cancer, the sun can never be overhead; if you are on the tropic, the sun is overhead only on the summer solstice. If you are south of the Tropic of Cancer, the sun can be north of overhead. If you are a slight distance south of the tropic, the sun is north of overhead at noon only a few days or weeks of the year. The farther south you are, the greater amount of time the sun spends north of overhead. 93 Revolution If you are on the equator, the sun spends the same amount of time north of the zenith as south of it. Open the Equator setting and step forward a day at a time by using the Single Step Forward button to track the suns location at noon each day, as seen from the Equator. When the Sun appears above the zenith on the screen, it is in the northern half of the sky, and when it appears beneath the zenith, it is in the Southern half of the sky. The sun is directly overhead at noon on the two equinoxes. As you head south of the equator, the sun spends more time north of overhead than south. It is exactly overhead, however, only twice a year. The line of latitude that lies 23-1/2 south of the equator - the Tropic of Capricorn - marks the southernmost line on the earths surface where the sun passes overhead. If you are south of this tropic, Sunshine and the Earths Circumference By the time of the ancient Greeks it was understood that the earth is a sphere and that its sphericity causes the sun to sit higher in the sky as seen from latitudes closer to the equator than from northern latitudes, where the sun shines down at a shallower angle. In the third century BC Eratosthenes noted that the sun shone vertically down a well in Aswan, Egypt, at the summer solstice, but on the same date it was 7 south of overhead at Alexandria, 800 kilometers (500 miles) to the north. He assumed that the distance to the sun was so great that its rays struck all parts of the earth on parallel paths, and correctly concluded that the distance from Alexandria to Aswan was 7/360 of the circumference of the earth. Section 3 - Earths Celestial Cycles 94 the sun is always north of overhead. The area of the earths surface between these two lines is called, simply, the tropics. These names - Tropic of Cancer and Tropic of Capricorn - reflect ancient history. When the tropics were named, the sun was in Cancer on the summer solstice in the Northern Hemisphere, and in Capricorn on the winter solstice. This is no longer the case, as you will learn in chapter 3.3. Arctic and Antarctic Circles The tropics mark the limits on the earths surface where the sun can be overhead. Two other lines on the earths surface mark the extreme points where the sun does not set - or does not rise. These are the Arctic and Antarctic Circles. Move to the Tropic of Cancer by opening the file WinterTropic. As with all points north of the Equator, the sun is at its lowest noontime elevation on the winter solstice. Double-click on the sun to bring up the Info Window, and read the suns elevation: 43. This is 20 higher than the suns maximum elevation as seen from Chicago on the same date. From the tropics, the sun is relatively high even at the winter solstice (which is why it is not cold in the tropics in winter). The farther north you are, the lower the sun is at noon. As we move northward along the Earths surface, the maximum elevation of the Sun at midday on the winter solstice decreases. For every degree of latitude we move north, the maximum elevation of the sun is one degree lower. At some latitude the suns elevation will be zero; on the winter solstice the sun will just barely rise before setting immediately again. This line of latitude - called the Arctic Circle - lies 23-1/2 south of the North Pole at a latitude of 66-1/2. If you move north of the Arctic Circle, the sun does not rise at all on 95 Revolution the winter solstice. The farther north you are of the Arctic Circle, the greater the number of days in the year when the sun does not rise. At the pole itself, the sun does not rise for six months! Lets position ourselves at the Arctic Circle just before noon on the winter solstice by opening the file ArcticWinter. Step through time at 10-minute intervals using the Single Step Forward button. The centre of the sun barely touches the southern horizon at noon before setting again. Now open the file ArcticSummer. You are viewing from the same location on June 21, the summer solstice. The sun is much higher in the sky - it has a noon-time elevation of 47. The suns rays do not warm the ground very effectively at such an angle, so it is safe to predict a cool June day for the Arctic Circle. Step forward through the rest of the day until sunset - and discover that there isnt one! The sun barely touches the northern horizon at midnight, but it does not set. You are in the land of the midnight sun - at least on this one day, the summer solstice. If you have ever traveled far north - to Alaska, Scotland, or Norway - in the summer, youve noticed that it gets dark very late at night or not at all. It is amazing to be able to read a newspaper at 11 p.m. However, if youve been at the same place in mid-winter youve been equally amazed that it remains dark until late morning and is not terribly bright even at noon; people often have to use lights in the house all day long in winter. The Arctic Circle is the southernmost latitude that experiences a midnight sun, and then only for one day - the summer solstice. As you move north of the Arctic Circle, there are more days surrounding the solstice when the sun does not set. At the North Pole, the sun rises on March 21 (the vernal equinox) and sets on September 21 (the autumnal equinox) and there are six months of midnight sun. That is followed by six months of no sun (but not of no light - Section 3 - Earths Celestial Cycles 96 it takes a long time for the sky to get dark once the sun has set at the north pole.) There is a corresponding latitude 23-1/2 north of the South Pole called the Antarctic Circle where all of this is true too, but six months out of phase. When the north polar regions are experiencing continuous sunlight, the south polar regions are in continuous darkness. When the sun rises at the North Pole, it sets at the South Pole. When thinking about seasons and daylight, the earth is very symmetrical about its vertical axis. Daylight Quiz Test your understanding. Which latitude has more hours of total sunlight during the year - the equator or pole? Think about it - you have enough information to figure it out. Answer: None of the above. Ignoring clouds, all parts of the earths surface receive the same number of hours of sunlight during a year, which is a total of exactly six months worth. At the equator, the sunlight comes in 12- hour batches every day with little variance from one day to the next through the seasons; at the pole, it comes six months at a time, all at once, from March through September. Why, then, is it colder at the poles? Because the suns rays always strike the poles at a shallow angle, while at the equator the sun shines down fiercely from nearly overhead 12 months of the year. 97 Revolution The Sun Also Rises But Where? So far we have been thinking about the suns height above the horizon at noon and how it changes during the year. You could monitor the suns height to track the seasons, and if you know the suns maximum altitude, you know the date. Let us now think about seasons in terms of the suns rising and setting points on the horizon. Open the file SunElevation2, and we are back in Iceland on the vernal equinox. It is 7:30 a.m. and the sun has just risen. Notice where on the horizon the sun rose; it rose due east, or 90 azimuth. Step slowly step through the calendar at one-day intervals for a few weeks, and stop. You will notice two things happening. Each day the sun is north of and higher than its position the day before. It is higher each succeeding day at the same time (e.g., 7:30) because it rises earlier, and we have already discussed why this is so; now we want to keep track of the position on the horizon where it rises. Ignore the suns changing altitude, and resume running forward through time. The suns rising point moves northward through the months at a decelerating rate until it reaches its northernmost rising point; it pauses its northward travel before beginning to move south again. Note the date - it should be the summer solstice, June 21. Bring up the Info Window to see that the sun rose at 3:02 a.m. Change the time to 3:02 a.m. to put the sun on the horizon, and find its azimuth by bringing up the Info Window again. On June 21, as seen from Iceland, the sun rises in the northeast at an azimuth of 21. The Info Window also shows that sunset will occur at 11:56 p.m. Change the time to this time and find the sun again on screen, setting in the northwest. Bring up its Info Window again. If you check its azimuth, you will find that it is just under 339, or 21 west of north (360 - 21 = 339). The suns setting position each day Section 3 - Earths Celestial Cycles 98 is always symmetrical with its rising position. If the sun rises 30 north of due east one day, it sets 30 north of due west that same day. At the summer solstice the suns rising point is as far north as it will ever get. On the following day, it begins to move south - slowly at first but at an increasing rate until the rate of change is greatest at the autumn equinox. It rises due east at the autumn equinox - as it does on the vernal equinox. For the next six months, the sun rises in the southeast and sets in the southwest. Continue stepping forward in time to see this change. You may want to occasionally set the time ahead as you step through the days in order to keep the sun near the horizon. Notice that by late November the suns rising point changes little from day to day, and the sun again stands still on the winter solstice, when it reaches its southernmost rising point. From Iceland this point has an azimuth of 156. This is as far south of due east (66) as the sun rose north of east on the summer solstice. As the winter solstice marks the suns lowest noontime elevation, so it marks the suns southernmost rising and setting points. Perform the same experiment for your home town and find the suns northernmost and southernmost rising and setting points. Because the sun rises at the same azimuth on the same date each year, if you know the suns rising point on the horizon, you know the date. (In practice, the sun rises at the same azimuth twice during the year - except at the two solstices - but a clever person can keep the two straight.) Earlier cultures knew this too, and many kept track of the date by watching the point on the horizon where the sun rose or set. In Russia earlier this century one person in each village was appointed to sit in a certain place and monitor the sunsets to know when the festival days should be celebrated, and some native Americans still use the technique. Finally, look at the suns changing and rising points from the Arctic Circle and then the North Pole and South Pole. Ask yourself 99 Revolution what you expect to see before doing the experiments. By now, you should have a solid understanding of how the Earths revolution around the Sun and its tilt combine to cause changes in the suns motion throughout the year as seen from all parts of the earth. A tool like Starry Night is invaluable for such visualizations. Seasonal Changes in the Stars and Planets While the changes in the motion of the sun are the most dramatic effects of the earths revolution, our ability to observe the stars and planets is affected as well. This is due to the suns apparent motion through the celestial sphere along the ecliptic. We saw in the exercise for chapter 1.2 that constellations are sometimes classified as winter or summer constellations depending on the season when they are best observed. The general rule of thumb is that the farther a constellation is from the sun, the longer it is visible at night. So when is the best time to see Scorpius? Not during the month of November, when the sun passes through it, but six months later, when the sun is on the other side of the sky. This rule of thumb works best for constellations near the celestial equator, for their distance from the sun varies the most with the changing seasons. The farther a star is from the celestial equator, the more difficult it is to group it The Scorpion is a prominent figure during evenings in the late spring and summer Section 3 - Earths Celestial Cycles 100 with a certain observing season. The motion of the moon and planets is more complex because we have to consider the apparent change in their position due to the earths revolution, as well as the true motion due to their orbit around the sun (or the earth, in the case of our moon). The motion of the moon is studied in chapter 4.1, and the motion of the planets is considered in chapter 4.2. 3.3 PRECESSION So far weve looked at two motions of the earth which change the appearance of the sky: the earths daily rotation on its axis and its annual circuit around the sun. The third motion of the earth which changes the appearance of the sky, and which has been mentioned several times, is precession, also called precession of the equinoxes. It is the wobbling of the earth on its axis. Precession was discovered in the second century BC by the Greek astronomer Hipparchus. He noticed a new star, which we would call a nova or supernova (an exploding star), and wondered about the permanence of the stars. Although it was not customary for astronomers to make actual observations of the sky (such work was considered labor, and manual labor was assigned to slaves), Hipparchus decided to make an accurate record of the stars, their brightnesses, and positions. He compiled the first complete star chart - one that was used for centuries after. Like all star catalogs of the time, it used the ecliptic co-ordinate system. In the process, he compared stars he observed with observations recorded 150 years Section 3 - Earths Celestial Cycles 102 previous by earlier astronomers, and he noticed a systematic shift in the ecliptic longitudes - but not latitudes - of the stars that was greater than could be accounted for by errors of measurement. He concluded that the co-ordinate system itself was shifting, although he could have no idea why. Since the equinoxes - the intersections of the celestial equator and ecliptic - were shifting, and the vernal equinox marked the zero point of the ecliptic co-ordinate system, he called the motion the precession of the equinoxes. His value for the annual Worshipping the Secret of Precession A mysterious religion, now extinct, once incorporated secret knowledge of the precession of the equinoxes. This knowledge was kept so private that only in the last few decades was it rediscovered. Yet at one time its followers were spread through the Roman Empire from England to Palestine and their religion was a rival to young Christianity. Worshippers of Mithras portrayed their hero slaying a bull in the presence of figures of the zodiac. Taurus, the celestial bull, died in the sense that precession had moved the location of the vernal equinox from Taurus into Aries, ending the Age of Taurus. A force that could move the equinox was stronger than any other yet known, for it moved the entire cosmos. Such a force must come from beyond the cosmos, and it was worshipped by the followers of Mithraism - who kept this knowledge a secret. They certainly would have been shocked to learn that the force originates with the pull of the sun and moon on the earths equatorial bulge! 103 Precession precessional shift was within 10% of the correct value. In addition to discovering precession and compiling the first star catalog, Hipparchus devised the stellar magnitude system that is still used today, and was the first to specify positions on the earths surface by longitude and latitude. He is justly remembered as the greatest astronomer of antiquity. Today we know that the earths axis wobbles and the equinoxes precess largely because of the gravitational influence of the moon. The earths relatively rapid 24-hour spin causes the earth to bulge at its equator. The earths equatorial diameter is 21 kilometers (13 miles) greater than its polar diameter. The moon - and to a lesser extent the sun and to a far lesser extent the planets - pull on this slight bulge. The bulge is oriented along the earths equator, but the moon and sun pull from a different direction - from the ecliptic. The effect is to try to pull the earth into a more upright orientation. But the earth is spinning like a gyroscope, and it resists being pulled over. Instead, it precesses, or wobbles; the amount of tilt (in this case, 23-1/2) remains constant while the direction of tilt changes. The earths axis traces a huge circle in the sky with a radius of 23-1/2 in a time span of 25,800 years (see illustration, right). That is a long time - there are as many human lifetimes in one wobble as there are days in one year. Starry Night can show a range of time from 4713 BC to 10 000 AD, a period of 14, 700 years, or just over half of a precession cycle. The celestial The Earth's axis rotates (precesses) just as a spinning top does. The period of precession is 25,800 years. Section 3 - Earths Celestial Cycles 104 equator, which is always 90 from the poles, wobbles at the same time and at the same rate as the poles. The earths axis meets the sky at the North Celestial Pole (abbreviated NCP), and as the axis precesses, the NCP changes. Right now, the NCP is less than 1 from the North Star, Polaris, and a century from now the NCP will be slightly closer to Polaris. Then the NCP will move on. Five centuries ago, when Columbus sailed the ocean blue, Polaris was 3-1/2 from the NCP, and sailors had to make allowances for this offset when navigating. At the time of the birth of Christ, Polaris was 12 from the NCP and not a useful pole star at all. Almost thirty centuries earlier, Thuban - a faint star in Draco - was the pole Thuban and the Great Pyramid The North Star at the time of the construction of the Great Pyramid was Thuban, an unassuming 4th-magnitude star in Draco the Dragon. You can find Thuban midway between Mizar in the Big Dipper and the end of the bowl of the Little Dipper. In 2700 B.C the earths axis pointed near Thuban, and the star held special significance to the Egyptians, who associated it, and the undying stars that were circumpolar and that never set, with the Pharaoh. The northernmost air shaft leading upward from the Kings Chamber in the Great Pyramid pointed to Thuban, symbolically connecting the dead pharaoh with the central undying star. Thuban is corrupted Arabic for serpents head. Open the file Thuban to see how close Thuban was to the skys pole when the pyramids were built, in 2560 B.C. 105 Precession star. Through the millennia, the earth has had several pole stars - but most of the time there has been no bright star near the NCP. Open the file Precession. We are in the year 1 AD, and Augustus is the first Emperor of Rome. Notice that the grid lines converge on the North Celestial Pole, which is near the Little Dipper. As you should know by now, Polaris is the bright star in the handle of the Little Dipper, and is to the bottom left of the pole. The North Celestial Pole is in the faint constellation Camelopardalis. Now step forward through time at 40-year intervals and watch the pole move towards Polaris. Pause near the year 1500 to notice that, to Columbus, the North Star was about three degrees from the skys pole (click on Polaris and drag the cursor to the North Celestial Pole to read the angular separation in degrees). Continue moving forward in time until you have reached the present. At present, the North Celestial Pole is less than one degree from the star Polaris. Reduce the time step to 5 years and zoom in until your field of view is about 10. Continue stepping forward in time to discover that the Pole will be closest to Polaris (about 1/2 apart) around the year 2100 before moving again. Restore your field of view to 100 and change the timestep to 100 years. Step forward through future millennia to discover future pole stars. Starry Night allows you to go as far forward as 10 000 AD. There will not be another pole star until about 4,000 AD when Er Rai (Gamma Cephei) will be fairly close. In 7,600 AD the star Alderamin (Alpha Cephei) will be The three best north stars of the next 10 000 years. This view is from the year 4100 AD. Section 3 - Earths Celestial Cycles 106 the north star. Our present pole star is the best the earth will ever have. No other star is ever as bright and as close to the celestial pole as ours will be for the next two hundred years. Its one of those little things we just take for granted. Open the file PrecessionSouth. We are at the earths South Pole at approximately the present date. The South Celestial Pole is overhead. Notice that it is in the constellation Octans, but it is not near any bright star and there is no South Star. Step forward through time to discover future South Pole Stars. You will see that four different stars will be excellent pole stars far in the future, including two (Aspidiske and Delta Velorum) which are comparable in brightness to Polaris. The skys pole shifts against the stars due to precession, but so does every point on the sky. The entire equatorial co-ordinate grid precesses with respect to the stars, as Hipparchus discovered. The RA and Dec of a star are valid numbers only for a particular moment, called the epoch and the co-ordinates change with time. For example, the co-ordinates of Vega in 2000 AD are RA 18h 37m, Dec +38 47', but in 2500 AD they will be RA 18h 54m, Dec +39 22. We must specify the epoch of the co-ordinates if we are dealing with long spans of time or extremely precise co-ordinates. Astronomers often use J2000 co-ordinates, which are the co-ordinates an object had in January 2000. Now you can explain why two different equatorial co-ordinate systems show up in an objects Info Window in Starry Night. One system gives the co-ordinates for the current date (current meaning whatever date you are looking at with Starry Night), the other gives the co-ordinates for January 2000. If you compare the two systems, the co-ordinates will be almost identical. Precession only has a major effect over a very long period of time. 107 Precession Precession & Astrology Precession has an interesting effect on astrology, and especially on birth signs or astrological signs. The signs - such as Scorpio - are each a uniform section of the sky 30 wide. They are measured eastward along the ecliptic from the vernal equinox, which is the intersection of the ecliptic and the celestial equator and is the zero point. When this system was set up around 600 BC, the zero point was in Aries and was called the first point of Aries. The constellation Aries encompassed the first 30 of the ecliptic; from 30 to 60 was Taurus; from 60 to 90 was Gemini, and so on. This scheme ignored actual stars, but uniformity was more important than fussing about star positions. Since then, precession has caused the celestial equator to wobble so as to cause the intersection point between it and the ecliptic to move westward along the ecliptic by 36, or almost exactly 1/10 of the way around. Open the file FirstSign and notice that the date is 600 BC. The intersection of the ecliptic and celestial equator is in western Aries. Step forward through time in intervals of 50 years and notice that the intersection moves westward. The ecliptic does not move, but the celestial equator does. The vernal equinox crossed from Aries into Pisces around the time of the birth of Christ (and the birth of Hipparchus), according to modern constellation boundaries. Stop in the year 2000 and notice that the intersection, which is still called the first point of Aries, has slipped most of the way through Pisces. In six more centuries it will leave Pisces. Your birth sign in your morning newspaper horoscope ignores precession. What your horoscope calls Aries is the 30 segment along the ecliptic that is east of the current location of the vernal equinox - but most of it is in Pisces! The next 30 segment, called Taurus in horoscopes, is largely in Aries. In the last 2,600 years Section 3 - Earths Celestial Cycles 108 the signs have slipped 2,600/25,800 of the way around the sky in a westward direction, relative to the stars beyond. The astrological signs are directions in space that do not correspond to the stars in the astronomical constellation with the same name. Precession causes the position of the sun on the vernal equinox to shift as the earth wobbles - but so does the position of the sun on every date. This means that it is not only the names of the Zodiac signs that are now inaccurate. The names of the tropics are now inaccurate as well. Open the setting Tropic, which shows the position of the Sun as seen from a viewing location along the Tropic of Cancer at the time of the summer solstice in the Northern Hemisphere, June 21. If you double-click on the Sun to bring up its Info Window, you will see that the sun has an altitude of almost 90, meaning it is directly overhead. But the Sun is not in the constellation Cancer! Use the Magnify button to zoom in and see that it is actually just inside the boundary of Taurus (it was in Gemini until only a few years ago). Similarly, on the Southern Hemisphere summer solstice on December 21, the sun is in Sagittarius, not Capricornus. Why is there a discrepancy? To find out, open the setting OldTropic. This is another view of the summer solstice as seen from along the Tropic of Cancer, but it is the year 150 BC, about the time that the tropics were named. You will see that the sun was in Cancer on the Summer Solstice then. Likewise, it was in Capricornus on the Winter The winter solstice has moved far from Capricornus in the 2600 years since the Tropic of Capricorn got its name. 109 Precession Solstice (Summer Solstice in the Southern Hemisphere), as you can also demonstrate with Starry Night, if you wish. Since 150 BC, precession has shifted the summer solstice westward from Cancer to Gemini to Taurus and the winter solstice from Capricornus to Sagittarius. Therefore, the Tropic of Cancer should be renamed the Tropic of Taurus, and the Tropic of Capricorn should now be named the Tropic of Sagittarius. But these names too will be ephemeral, and the tropics will continue to shift. In another 23,200 years the cycle will have been completed and the sun will once again appear in Cancer and Capricornus on the solstices. This table lists the dates when the sun is actually within the astronomical constellations of the zodiac, according to modern constellation boundaries and corrected for precession (these dates vary by up to one day from year to year). You will probably find that you have a new birth sign. If you were born between November 29 and December 17, you are an Ophiuchus! Constellation Dates Capricornus January 20 to February 16 Aquarius February 16 to March 11 Pisces March 11 to April 18 Aries April 18 to May 13 Taurus May 13 to June 21 Gemini June 21 to July 20 Cancer July 20 to August 10 Leo August 10 to September 16 Virgo September 16 to October 30 Libra October 30 to November 23 Scorpius November 23 to November 29 Ophiuchus November 29 to December 17 Sagittarius December 17 to January 20 Section 3 - Earths Celestial Cycles 110 To confirm that these dates are correct, open the file RealZodiac. Step forward through a year and notice the dates on which the Sun moves into a new constellation. The Dawning of the Age of Aquarius According the popular 60s song, it is the dawning of the Age of Aquarius. What does this mean, and when does that happy day arrive? The name of an astrological age comes from the constellation the vernal equinox is in. During classical Greek times, the vernal equinox was in Aries and it was the Age of Aries. By about the time of the birth of Christ, the equinox had precessed westward until it stood in Pisces, and the last 2,000 years has been the Age of Pisces. When the equinox moves into Aquarius, the Age of Aquarius will begin. Using modern constellation boundaries, the equinox has 9 farther to precess before it enters Aquarius, and that wont happen until the year 2597. Apparently we have a bit of a wait before the dawning of the age of universal peace and love. Open the file First Sign again, reduce the timestep, and step through time to confirm the year when the Age of Aquarius dawns. When will the Age of Capricornus begin? The Age of Sagittarius? 111 Precession The Extended Constellations of the Zodiac Using modern constellation boundaries, the sun travels through the traditional 12 constellations of the zodiac plus Ophiuchus. The moon and planets orbit on paths that are inclined to the ecliptic, and they travel through additional constellations. Not counting Pluto, whose orbit is inclined a whopping 17 and which would add several more constellations to the list, these are the 21 astronomical constellations that are visited by at least one of the solar systems major bodies. Aquarius Corvus Libra Sagittarius Virgo Aries Crater Ophiuchus Scorpius Cancer Gemini Orion Scutum Capricornus Hydra Pegasus Sextans Cetus Leo Pisces Taurus Open the file ExtendedZodiac and step through time to follow the motion of the moon. You can see that it oscillates around the line of the ecliptic. Imagine making a band which is twice the width of the maximum distance from the moon to the line of the ecliptic. If you wrap this band around the ecliptic line, every constellation partially or fully covered by this band is touched at some point by the moon. The naked eye planets that travel farthest from the ecliptic are Venus and Mars, so they make the biggest bands and travel through the most constellations. Blank Page Section 4 Our Solar System PEOPLE who live in brightly-lit cities cannot experience the awesome beauty of a dark night sky with its millions of stars and the Milky Way unless they travel away from home - but they can see the moon and planets. Tracking the motions of the moon and planets is perhaps the oldest astronomical activity and it is one that city-folk can still fully participate in. It is a good way to remain connected to the sky. Starry Night excels at showing you how the moon and planets move and where they will be in the future, and it lets you plan your skywatching and understand what is going on. Chapter 4.1 will look at the motions of the moon, while Chapter 4.2 will cover the motions of the planets. It is important to remember that the apparent motion of a moon or planet as seen from Earth is a combination of two things: 1) the motions of the earth (as discussed in the last three chapters) 2) the real motion of the moon or planet around its parent body 4.1 THE MOON As long as people have been looking up, they have been fascinated by both the motions and the changing appearance of the moon. Scholars debate how much was known when and to what use monuments such as Stonehenge and others were put, but all agree that the moon was monitored carefully and even worshipped from remote prehistoric times. We dont worship it today, but we can still follow its motions. The moon revolves about the earth approximately 12 times per year or approximately once every 30 days. This relatively rapid rate makes the moons motion among the stars easily visible from night to night. Because the solar system is relatively flat, the moon and planets stay near the suns path along the ecliptic. The moons orbit is tilted 5 to the ecliptic, and so the moon remains within 5 of the ecliptic. Half the month it is north of the ecliptic and half the month south, and it crosses the ecliptic at two points called the nodes. Of course, Starry Night will show you precisely where the moon Section 4 - Our Solar System 116 is tonight or any other night (or day), and not only from your backyard but from any place on earth. It is in almost the same position in the sky for everyone on earth at the same moment, but not exactly. Observers separated by a large distance will see the moon in a slightly different position relative to the background stars. This is an example of parallax - the apparent shift of a comparatively nearby object seen against distant objects when viewed from different positions. Demonstrate by using Starry Night to view the moon from two different cities at the same moment. Open the file MoonParNY file, and notice Saturn just above the moon. The time is 7:30 p.m. on November 11, 1997, and you are viewing from New York City. Leave this window open and now open MoonParDetroit, which shows the moon and Saturn as seen at the same moment from Detroit. Align the two Starry Night Windows so that you can see both views at the same time. Notice how Saturn - and the background stars - are in slightly different positions relative to the moon, depending on which city you are viewing from. In reality, it is the moon whose position has shifted relative to the far more distant planet and stars. Moving to the far side of the earth can cause the moon to shift its position by an amount greater than its own diameter! This is especially important in a solar eclipse or the occultation of a planet The Oldest Moon Observation A series of notches cut into a reindeer bone found buried in a cave in France may be a symbolic tally of the nightly appearances of the moon over a two-month span. This bone is 30,000 years old, suggesting that as early as the Ice Age people kept track of the position of the moon and counted the passing of time in months. 117 The Moon or star by the moon - the moon may align with the sun or occult a planet as seen from one city, but not from another. New & Full Moon The moons monthly orbital cycle begins at new moon, when the moon is in line with the sun. In reality the moon is usually a bit above or below the sun then and not directly in front of it, so new moon is defined as the moment when the sun and the moon have the same longitude, which is also approximately when they have the smallest angular separation. After one week, the moon has moved a quarter of the way around the earth and is at its first-quarter position. One week later finds the moon opposite the sun (or at least at longitude of the sun plus 180), and this is the full moon. One week later the moon is three-quarters of the way around its orbit, and we call it a third-quarter moon or last-quarter moon. After another week it has completed its cycle and is new again. How Long is the Moon Full? The moon is full when it is opposite the sun (when its longitude is 180 greater than the suns). It is full for only an instant, just as it is midnight (or 3:00 p.m.) for only an instant. That instant can be calculated to the nearest second, and it is the same moment simultaneously all over the earth. To most people, however, the moon looks full for three days to a week. For practical purposes, the moon is considered to be full for the entire night closest to the instant of its opposition. Section 4 - Our Solar System 118 The Month The time it takes the moon to complete one orbital cycle is a month, but how long is that? By now, you are probably not surprised to learn that there are two common definitions of the month. Open the file MoonMonth to find out why. Begin with the new moon on April 12, 2002. The time is 9:22 a.m. - the actual moment when the moon is new - and we are viewing from Honolulu, Hawaii. The moon and sun are very close together. The stars are turned on and daylight colors are turned off. Notice that the moon is just a fraction of a degree to the right of the star HIP 6751. Now step forward through time at one-day intervals until the moon approaches HIP 6751 again. This will occur on May 9, which is a period of 27 days. The precise orbital period is 27.32166 days -the time the moon takes to circle the earth once relative to the stars, and it is called a sidereal month. But notice that the moon is not yet new! It is a thin waning crescent. During the 27-1/3 days it took the moon to return to the same part of the sky, the sun has moved on 27 eastward. Recall that the sun moves eastward along the ecliptic because the earth moves around the sun, and that the sun moves eastward with respect to the stars at the rate of very nearly 1 per day. In order to complete its cycle of phases and become a new moon again, the moon has to move that additional 27-1/3, and that takes it 2-1/6 days. Step forward in time to see that the moon and the sun come closest together (and the moon is completely dark) on May 12. This cycle of the moons phases - from new moon to new moon - has a period of 29.53059 days and it is called a synodic month. It is what we normally think of as a month. The month is a fundamental cycle in the sky, and since prehistoric times it has formed the basis of a calendar. Early calendars were lunar - they were based on actual observations, and a new 119 The Moon month began with the sighting of the new moon. This is still true in Islamic culture. A problem is that there are not a whole number of lunar months in a year - there are 12-1/3 synodic months in a year. A calendar based on synodic months has to add leap months periodically if it is to keep the months in phase with the year. The Islamic calendar does not have leap months, so its months (and holy days) do not always fall in the same season. Our modern Gregorian calendar is solar, and months are given arbitrary lengths so they add up to 12 months of 365 days total. The moons cycle also forms the basis of our week, which is one quarter of a synodic month (rounded off to the nearest whole day). The idea of the 7-day week originated in Babylon and was spread around the Mediterranean world by the Jews after they were released from Babylonian captivity. Motion of the Moon Because the moon stays near the suns path, if you know the movements of the sun you already know the approximate movements of the moon. There are differences, however. The new moon rises and sets with the sun and follows its path across the sky, but at its other phases the moon travels a different path than the sun. Lets look at a new and then a full moon to see why this is so. Open the file MoonPath; the time is 7:00 a.m. on October 27, 2000, and the sun and new moon are close together and just above the southeastern horizon. Daylight is turned off so we can actually see both of them. Notice where the sun and moon are on the horizon as they rise. Step through time in 10-minute intervals and watch them move together across the sky until they set. Note the direction in which they set. Now advance time by two weeks to setting the date and time to 5:30 p.m on November 11. The moon is now full, and it is again Section 4 - Our Solar System 120 just above the eastern horizon. Notice how far north it is of its rising position two weeks earlier. Advance through time at 10-minute intervals and watch it move across the sky, and confirm that it sets far to the north of its setting position two weeks earlier. The full moon is 180 opposite the sun and does not follow the suns path for that day - it follows the path the sun will have in six months, when the sun is 180 in advance of its present position. The path of the full moon is six months out of phase with the path of the sun; the full moon in winter travels the path of the sun in summer, and vice versa. The sun is highest in the sky (in the Northern Hemisphere) at the summer solstice in June, but the full moon in June travels on its lowest path for the year. The full moons in December and January pass the highest overhead at midnight. Close the file MoonPath. Midnight Moon If you are north of the Arctic Circle, the sun can remain above the horizon for more than 24 hours and you can see a midnight sun (see chapter 3.2). The moon too can remain above the horizon for more than 24 hours in a row, and at other times it does not rise at all! Open the file MoonArctic1 to view the midnight moon from northern Greenland. Step forward though time in 1-hour steps to watch it circle the horizon; at what time will it be due north? when will it finally set? Because the moon traces the suns yearly path in only a month, the amount of time it spends above the horizon varies much more rapidly than does that of the sun. Close the file MoonArctic1. 121 The Moon Surface of the Moon Through even a small telescope you will see more features on the moon than on the sun, all the planets, and all the comets and deep space objects put together. It is beyond the scope of this book to interpret the moons features (consult a popular observing guide to the sky, or visit www.livesky.com ), but let it be said that the moons surface is the result of impacts from above and lava flow from below. During its first half-billion years, the moon was struck again and again by asteroids - debris left over from the formation of the planets. They exploded on impact as their kinetic energy was changed into heat, and they vaporized themselves and a great deal of the rock that they hit, blasting out huge craters. By four billion years ago the moons surface was so saturated with craters that not one square centimeter hadnt been hit at least once. Over the next billion years, immense amounts of hot lava flooded the low areas with smooth lava flows that cooled and turned dark. These colossal lava flows formed what we call the lunar seas - seas of cooled lava. On earth, erosion from wind and rain has erased the evidence of the earths violent early history; but the moon has no atmosphere and therefore no erosion. The lava stopped flowing long ago and few asteroids fall today, so the moon looks much like it did three billion years ago. It is a very common misconception that the moon has a fixed dark side and a fixed light side. The sun rises and sets on the moons surface as it does on the earth, but with a day that is 27-1/3 earth-days long. Note however that the moon does keep one side permanently Sunrise as seen from the moon, with a new earth just to the suns right. Section 4 - Our Solar System 122 The Earth from the Moon Everyone has seen the moon from the surface of the earth, but only 12 men - the Apollo astronauts - have seen the earth from the surface of the moon. You can join them with Starry Night. Open the file MoonApollo11. You are at the Apollo 11 landing site along the moons equator on July 20, 1969, the day of the moon landing. The earth is high above the western horizon, in the constellation Pisces. Double-click on the earth to bring up an Info Window to see that its phase is 68%; it is a gibbous earth. You cannot see the moon half the time from your home on earth, but there is no need to wonder if you will see the earth from the moon. If you are on the side of the moon that faces the earth, the earth is always in the sky, day and night, and it never rises nor sets. It changes position little from day to day and year to year, but it does change its phase. The earth goes through a full set of phases once a month just as the moon does, but the two have opposite phases and remain 180 out of synchronization. When the moon is new, the earth is full, and vice versa. Think about it a minute, and then step forward through time with Starry Night. You will see the earth rotate and the stars and constellations move behind it, but the earth hardly changes its position in the sky. If you wish to spend more time exploring this concept, open multiple windows to view from the earth the moon is new (as seen from earth), for example, it is in the direction of the sun; at that same moment the earth (as seen from the moon) must be in the opposite 123 The Moon turned toward the earth while the back side (or far side) of the moon is permanently turned away from earth (i.e., its rotation and revolution periods are the same). The moon is in what is called locked rotation with the earth (as are several other moons in our solar system), and it is a long-term result of tidal forces between the earth and moon. All the familiar seas and craters are on the moons near side; the far side was unknown until photographed by a Soviet spacecraft in 1959. Eclipses In the first section of this book we saw how Starry Night was able to simulate a solar eclipse. Both solar and lunar eclipses are caused by the motion of the moon around the earth. When the moon moves directly in front of the sun, there is a solar eclipse. When it passes exactly opposite the sun in the sky, there is a lunar eclipse. Eclipses are among the most awesome spectacles in nature, and one of the first duties of astronomers thousands of years ago was to predict eclipses - or at least to be on hand to perform the rituals that would cause the dreaded eclipse to hurry up and end! Only since about 600 B.C. could astronomers predict eclipses with any accuracy. With Starry Night, you can see eclipses far into the future and re-create eclipses that happened in the distant past. If direction as the sun. The earth is never visible from the far side of the moon - the side that permanently faces away from earth. Just as this side cannot be seen from earth, visitors to it will never see the earth in their sky. Section 4 - Our Solar System 124 you select Sky | Interesting Events, you will see a list of all the eclipses in the 20th and 21st centuries, and a description of whether the eclipse was solar or lunar, and whether it was total, annular or partial. Choose Best View to view the eclipse from the prime location or Local View to see what it looked like at that time for your home location. If you do search for eclipses, you will notice that they do not happen randomly in time. The moons path is inclined 5 relative to the suns path, and an eclipse can happen only when the sun is at or near a node of the moons orbit - one of the two places where the moons orbit crosses the ecliptic. When the sun passes a node, at least one solar eclipse must happen, and there can be two. The sun spends 38 days close enough to the node that the moon can pass in front of it. If the moon eclipses the sun during the first 8 days of that period, the eclipse will be partial and the moon will return and eclipse the sun again one lunar month later, yielding two partial eclipses. If the moon does not eclipse the sun until after the 8th day, there will be just one solar eclipse, and it will be total or annular, if seen from the best viewing location. Open the file EclipseNodes to see the geometry of the eclipse nodes. Two lines are marked. One is the path of the moons orbit; the other is the ecliptic, the line which traces the suns apparent path. The intersection point of these two lines is an eclipse node, and it is just to the bottom left of the sun. Step forward through time in one day intervals and you will notice that the sun is moving closer Eclipses can only occur when the sun is near the intersection of the ecliptic and the moons orbit. 125 The Moon to the node. At the same time, the moon is rapidly moving around the sky, but it is far enough away from the sun that it is off screen. On August 8, the moon comes into view. At this time, the sun is almost exactly at the eclipse node, so you can predict that there will be a solar eclipse when the moon passes by the sun. Continue stepping forward in time until August 11, when the moon and the sun line up. As predicted, there is an eclipse. It is a total eclipse if viewed from this location on Earth, which is Munich, Germany. Thousands of years ago astronomers realized that a solar eclipse could happen only during the 38-day period when the sun was near one of the moons nodes, and this interval was called the eclipse season. Before astronomers were capable of predicting eclipses reliably, they issued warnings that an eclipse could happen during the eclipse season - just as today hurricane warnings are issued in the Caribbean during the storm season. The predicted eclipse did always occur, but it was often visible only from a distant part of the earth and they were ignorant of it (they counted it as a miss). Today the only warnings astronomers issue regarding eclipses is to not Eclipses in Ancient History Ancient eclipses are important to historians because they allow us to date events with precision. The dates of many important events are poorly known, often only to the nearest decade, but the times of all solar and lunar eclipses can be calculated to the nearest minute. If an eclipse accompanied an historic event, the time of the event can be pinned down precisely. Eclipses provide crucial anchor points in history. Section 4 - Our Solar System 126 look at the sun without proper equipment. As with the solar eclipse, both the sun and the moon must be near an eclipse node for a lunar eclipse to occur. During a solar eclipse, the sun and the moon are at the same node, but during a lunar eclipse, they are at opposite nodes, which are 180 apart in the sky. Open the file EclipseNodes2 to see this. You are looking at the full moon, which is dark and in shadow because we are witnessing a lunar eclipse. Double-click on the moon to open its Info Window and read its azimuth. Now scroll around in the sky to find the sun. You will see that it is also at a node. If you open its Info Window, you will see that its azimuth is about 180 removed from the moons. There must be at least one lunar eclipse each eclipse season and it can be total, or there can be two - but then both are partial. Eclipse season is the same for both solar and lunar eclipses. After a solar eclipse, the moon must move about 180 in the sky for a lunar eclipse to occur. The moon makes a complete 360 circle in about four weeks, so it moves about 180 in two weeks. We would therefore expect lunar and solar eclipses to be separated by about two weeks and indeed, this is the case. Every eclipse season brings at least one solar eclipse and at least one lunar eclipse. If the moon crossed the suns path at the same point each year, eclipses would happen on the same date each year, but it doesnt, so they dont. The moons orbit regresses in a motion that is very similar to the precession of the equinoxes (see chapter 3.3). The moons two nodes precess westward along the ecliptic at the rate of 18.6 per year. The sun moves 18.6 along the ecliptic in 18.6 days and arrives at the node earlier the next year, causing eclipse season to move backward through the calendar. The eclipse of December 14, 2001 is followed by an eclipse on December 4, 2002. 127 The Moon The Last Total Solar Eclipse The earth has enjoyed total solar eclipses since the moon was formed, but our distant ancestors will not see any. Because of tidal friction with the earth, the moon is receding from us at the stately rate of 4 centimeters (1.5 inches) per year - about the same rate that the continents drift or that your fingernails grow. The moons apparent diameter is shrinking as its distance increases. In about the year 600,000,000 AD the moon will totally eclipse the sun for the last time; after this time the moon will be so distant that its disk will no longer be large enough to completely cover the sun, and people will see only annular and partial eclipses. Viewing a Lunar Eclipse A lunar eclipse is a direct (although less visually spectacular) counterpart of a solar eclipse. The sun is eclipsed when the moon moves in front of it and we find ourselves in the moons shadow. The moon is eclipsed when it moves into the earths shadow. This happens only at full moon - when the moon is directly opposite the sun. The earths shadow - like all shadows - has a central umbra and an outer penumbra. The umbra is the place where, if you stand there, the sun is totally blocked and the eclipse is total. The penumbra is the place where the eclipse is partial. A lunar eclipse has stages that resemble a solar eclipse, and tables of eclipse circumstances will list the times of the several contacts (when the moon begins to enter the penumbra, is fully within the penumbra, enters the umbra, Section 4 - Our Solar System 128 is fully within the umbra, mid-eclipse, begins to leave the umbra, begins to leave the penumbra, and is finally out of the penumbra). Starry Night will show these several stages graphically and you can determine the timings from the date/time display. Open the file LunarTotal and step through time to view a total lunar eclipse. Note the faint penumbra and the darker umbral shadow which the moon passes through. During a shallow penumbral eclipse, when the moon passes through only the outer edge of the earths shadow, the moon may not darken noticeably. In a deep penumbral eclipse the darkening is barely noticeable even to a careful observer. In an umbral eclipse, one portion of the moon grows dark and a casual observer would notice that something is amiss. Only during a total eclipse does the moon darken substantially and take on a reddish or orange color. This coloring comes from light refracted around the edge of the earth towards the moon; it comes from all the earths sunrises and sunsets. If the earths atmosphere is especially opaque, as happens following a major volcanic eruption, the eclipse can be so dark that the moon disappears, but this is rare. Generally the moon takes on a deep coppery color, dims as the stars come out, and looks very pretty. To superstitious people in former times, the red color of the moon made it an unpleasant and fearsome omen. The Greeks Proof of a Round Earth The ancient Greeks knew that the earth is a sphere by observing lunar eclipses. They saw that the edge of the earths shadow on the moon always has a circular shape, and they knew that the only object that only casts a circular shadow is a sphere. They did not, however, know the size of the earth or moon or the distance to the moon. 129 The Moon The earth casts a much larger shadow on the moon than the moon does on the earth, because of the earths larger size. This means that many more people will see a total lunar eclipse than a total solar eclipse for three reasons: 1) total lunar eclipses are slightly more common than total solar eclipses (there are about 3 total lunar eclipses for every 2 total solar eclipses) because the earth, moon, and sun do not have to be exactly in a straight line for a total lunar eclipse to occur. 2) most total lunar eclipses are total as seen from almost anywhere on earth where the moon is above the horizon at the time of the eclipse, while total solar eclipses are only total as seen from a narrow path. 3) total lunar eclipses generally last much longer than total solar eclipses. Unlike a solar eclipse, you need take no special precautions to observe a lunar eclipse. The moon is a dark rock sitting in space, and when eclipsed it just grows darker still. Use binoculars or a telescope to bring out subtle coloring. Occultations Another interesting event to observe with binoculars or a telescope is the occultation of a planet or bright star by the moon. As the moon circles the earth, it occasionally passes in front of a more distant object and eclipses it. If the object eclipsed has a much smaller apparent size than the moon (a planet or star, for example, but not the sun), then the preferred astronomical term is occultation, from the Latin root occult, which has nothing to do with the supernatural but simply means to conceal or hide. Section 4 - Our Solar System 130 A star, whose apparent diameter is miniscule, blinks out instantaneously when the moons edge moves in front of it, and it is startling how abruptly a star disappears from sight. Only stars within 5 of the ecliptic can be covered by the moon, so we can only see star occultations for a handful of bright stars (Spica and Regulus are the most prominent). A planet takes several seconds to disappear as the moon moves in front of it. Up to an hour and a half later, the star or planet reappears from behind the other side of the moon. Depending on the moons phase, the leading edge will be light while the trailing is dark (full moon to new), or vice versa (new moon to full). All planets eventually cross the ecliptic, so any planet can be involved in a planetary occultation. Both types of occultations are shown in the files Occultation1 and Occultation2; Open these and step forward through time slowly to watch Saturn and the star Spica respectively disappear and then reappear. The more readily visible occultations are listed in monthly sky calendars such as the one appearing in Sky & Telescope magazine, and at the International Occultation Timing Associations web page (see www.livesky.com for the link to this site). 4.2 PLANETS, ASTEROIDS & COMETS Planets Chapter 1.3 of this book showed us a few fundamental aspects of the planets in our solar system: the earth is a planet like the others in motion about the sun; all of the planets (with Pluto the major exception) revolve around the sun in the same plane; and the planets move in the same direction but at different speeds. Humans have required a significant fraction of their collective history to arrive at these ideas. It was especially difficult to discard the idea of the stationary earth. In the exercise for chapter 1.3, we viewed the motion of the planets with the sun at the centre, which we now know is the correct view. But how would the planetary motions look if the Earth was Section 4 - Our Solar System 132 really stationary? To find out, click on the Go menu and select Earthcentric. Now the motions are not so simple! The sun makes a regular loop around the earth, but the other planets have more complicated motions. It is easy to see why it was thought from the time of the ancient Greeks until about 1600 AD that the planets move on epicycles - circles on circles - as they orbit the earth. As seen from Earth, each planet repeats a regular pattern in the sky. This cycle can be thought of as beginning when the earth, sun and the planet are all in a straight line, with the planet on the opposite side of the sun from the Earth (this alignment is called superior conjunction), and ends the next time they come together in the same Keplers Laws Prior to the work of Johannes Kepler (1571 - 1630), no one understood how the planets orbit the sun or the relationships between their orbits. Kepler had the good fortune to inherit the exceptionally precise planetary observations of his teacher, Tycho Brahe, and they allowed him to devise his three laws of planetary motion. They summarize how the solar system is put together and how it operates. In simple terms, the laws state: 1. Planetary orbits are ellipses with the sun at one focus. 2. Planets orbit fastest when closest to the sun. 3. The length of a planetary year is related to its distance from the sun by a simple formula. Before Kepler, planet motions were a mystery; after Kepler, the planets positions could be predicted accurately far into the future. 133 Planets, Asteroids & Comets alignment. The length of this cycle depends not on the period of the planets orbit, but on the relative motions of both the earth and the planet about the sun. Mars actually takes the longest to complete its cycle, about 26 months from superior conjunction to superior conjunction. During this cycle, a planets phase and brightness will both vary, as will the best time to observe the planet. But how much these quantities vary depends on the particular planet. So lets take a look at the cycles of several different planets now. The Inferior Planets Well begin with the two inferior planets, to use a term that sounds disparaging but that is only a holdover from an era when all that distinguished one planet from another was its orbit. The inferior planets are Mercury and Venus, and they stay in the vicinity of the sun as seen from the earth. They can appear to the east (or left) of Orbit of Mercury or Venus Earth's orbit Sun Earth Inferior Conjunction Superior Conjunction Greatest eastern elongation Greatest western elongation Important points along the orbits of Mercury and Venus, the inferior planets. Section 4 - Our Solar System 134 A planetary transit of Venus Planetary Transits When Venus or Mercury is at inferior conjunction, it may make a transit in front of the sun. Planetary transits are actually a type of partial solar eclipse! Mercury and Venus are too small to eclipse the sun when they move in front of it, and we see them cross the suns surface as slowly moving black dots. Mercury appears the size of a small sunspot and Venus the size of a large sunspot, so transits cannot be observed without properly filtered telescopes capable of fairly high magnification. Just as solar eclipses happen only during eclipse season (see chapter 4.1) when the moons orbit is so aligned that the moon can pass in front of the sun, so too planetary transits happen only when the planets orbit is aligned so that the planet can eclipse the sun as seen from earth. For this reason, transits are rare, and they occur in sets separated by wide intervals of time. Transits of Mercury occur alternately in May or November in 2003, 2006, 2016, 2019, 2032, 2039, etc. They last up to 7-1/2 hours. The only two transits of Venus in the 21st century are in June 2004 and June 2012. If you are lucky enough to watch a transit, you will see the planet slowly creep across the sun from east to west, passing any sunspots that are present as it goes. Open the file VenusTransit and step through time to watch the transit of Venus in 2004. Change the location to see if it is visible from your home town. 135 Planets, Asteroids & Comets the sun, in which case they are visible in the evening sky; to the west (or right) of the sun, in which case they are visible in the morning sky; behind the sun, in which case they cannot be seen (superior conjunction); or at inferior conjunction when they lie between the earth and sun (which no other planets can be), and in which case they also cannot be seen. The ease with which these planets can be viewed is in direct correspondence with their angular separation from the sun. The farther a planet appears from the sun, the longer it is above the horizon during the night. Venus gets far enough from the sun to be conspicuous much of the year, but Mercury never travels far from the sun, so it usually cant be seen at all. Mercury never strays more than 28 from the sun. As seen from the earths equator it never sets more than two hours after the sun or rises more than two hours before it. This number is smaller the farther you are from the equator, making Mercury even harder to see from mid-latitudes. The bottom line: Mercury is seen only near the horizon during twilight. It is best to look for it with binoculars and it is most easily found when the thin crescent moon is near it to guide the way. Open the file Mercury to follow Mercury through several orbital cycles, beginning with the planet at superior conjunction (on the far side of the sun) on May 18, 2006. The orbital path which Mercury makes around the sun is shown. Mercury is so nearly in line with the sun that you will have to zoom in to see it. In reality, it rarely is exactly behind the sun (just as it rarely passes exactly in front of the sun) because its orbit is tilted 7 to the ecliptic. This is more than any planet but Pluto. Mercury is almost invariably north or south of the sun when in conjunction with the sun. Center and zoom in on Mercury to confirm that it is a tiny but full disk (you will have to zoom in very close, to less than 1'), and then zoom back out to the standard 100 field of view. Section 4 - Our Solar System 136 Step through time in one-day intervals and watch Mercury swing around to the left (or east) of the sun. Its angular separation from the sun increases daily until it reaches its maximum angular separation from the sun on June 23. This is called its greatest eastern elongation, which can be as little as 18 or as much as 27. Zoom in on Mercury to verify that it is at third-quarter phase. The planet is so tiny - typically 5 arcseconds in diameter - and so near the horizon that its phase is hard to see even with a good telescope. Restore the field of view to 100 and change the time to 8 p.m. to see that it shines near the western horizon. Change the time step to 3 minutes and step forward in time to watch it set at 9:43 pm. Mercury is visible only during a narrow window of time, after the sky becomes dark enough to see it, but before it sets. Change the time back to 12:00 p.m. and the time step to 1 day. Start time running forward again. Mercury remains near its greatest eastern elongation for a few days, and its position in the sky changes little because it is moving nearly directly towards the earth then, although it often moves a short distance north or south (in this instance) in a shallow arc. Then Mercury begins to move westward toward the sun again. When it passes the sun, it is on the near side of the sun and much closer to the earth than when on the suns far side, and Mercurys apparent speed across the sky is greater than when it was on the far side of the sun. It is at inferior conjunction, when it is most nearly between the earth and sun, on July 18, and this is the date when it officially passes from the evening to the morning sky. It is now at its new phase (zoom in to verify, and then zoom out again), but you wont see it in the sky because it rises and sets with the sun. A week later it reappears as a morning star near the eastern horizon during dawn. Telescopically it is a large (by Mercury standards) thin crescent. It climbs higher each day until it reaches its greatest western elongation on August 7. This coin- 137 Planets, Asteroids & Comets cides closely with its earliest rising time and its best pre-dawn visibility. At its greatest elongation it is at first-quarter phase. Mercury then moves more slowly back towards the sun and disappears before it is in line with the sun at its superior conjunction on September 1, when it is full again. Mercury reveals little through a telescope. Ironically, it is one of the three planets whose surface is visible (Mars and Pluto are the others; the rest of the planets are shrouded by clouds), but it is too small and too distant for any surface features to be seen. Only its phase might be seen, and although it changes phase rapidly, the blurring effect of turbulence in our atmosphere (a problem with all objects observed at low altitude) makes it hard to tell what phase it is even through a good telescope. Venus goes through a cycle similar to Mercurys. Open the file Venus and step through time in 3-day steps to follow a typical cycle of Venus, beginning at superior conjunction on October 27, 2006. Note how the orbital path of Venus takes it much farther from the sun than Mercurys path does. Zoom in at any time to see the phase of Venus. When it is on the far side of the sun it is small (about 10 arcseconds in diameter) and full. The earth and Venus are moving around the sun at similar speeds (30 vs. 35 km per second, or 18 vs. 22 miles per second respectively), so Venus increases its angular separation with the sun only slowly. When it is 10 or more from the sun, it can first be seen as the evening star low in the west after sunset. Its great brilliance makes it easy to spot despite its low height. It is now waxing gibbous. Months later it reaches its greatest eastern elongation (June 9, 2007), which can be as great as 47 from the sun. At this time in its cycle, Venus sets after 11 p.m. at mid-northern latitudes and remains beautiful in the west long after twilight ends. It remains near its eastern elongation for weeks while it shines brightly late into the evening; it is then heading Section 4 - Our Solar System 138 towards the earth and moving very slowly against the stars. During the next several weeks it catches and then passes the earth on an inside orbit, quickly moving toward inferior conjunction with the sun (August 18, 2007). During these final weeks of its evening appearance, it becomes an increasingly thin and increasingly large crescent up to an arcminute in diameter - so large that its phase can be seen in very good tripod-mounted binoculars. It then seems to drop out of the evening sky, and in one month its visibility drops from very conspicuous to hard to see. After passing inferior conjunction, Venus quickly reappears in the morning sky. It gains altitude rapidly day by day until it reaches its greatest western elongation (October 28, 2007), when it is halffull. Its disappearance from the morning sky is as slow as its appearance in the evening sky because it is on the far side of the sun and its motion relative to the sun is slow. Venus is by far the brightest planet. Its brilliance comes from Galileo and the Phases of Venus Galileo discovered the phases of Venus with one of his first telescopes, demonstrating that Venus orbits the sun, rather than the earth. If Venus orbits the sun, so could the earth, and this was important proof of the Copernican theory of a sun-centered solar system, which until then had been a mathematical curiosity. Galileo published his observations in code to establish the priority of his discovery until he could confirm it. The file VenusGalileo shows Venus as Galileo saw it from Florence, Italy, in October 1610 through his telescope. It was near full and quite small, and it was not until months later that Galileo could easily discern its non-round shape. 139 Planets, Asteroids & Comets thick and highly reflective clouds that permanently shroud the planet. The surface features of Venus cannot be seen through any telescope. In fact, the clouds themselves are featureless in wavelengths visible to the eye. The planet is so brilliant it can be seen during the daytime if you know exactly where to look. The Superior Planets The other planets, Mars through Pluto, remain outside the earths orbit and are called superior planets. They are only superior in an orbital sense. Unlike the two inferior planets, the superior planets never come between the earth and sun. Also unlike the inferior planets, whose visibility is limited to the hours just before sunrise or just after sunset, the superior planets can lie opposite the sun and can be visible at any time of the night. Earth's orbit Orbit of Mars, Jupiter, Saturn, etc. Sun Earth Conjunction Opposition Important points along the orbits of the superior planets - Mars through Pluto. Section 4 - Our Solar System 140 Open the file Mars to follow one Martian cycle. We begin with the planet at superior conjunction on the far side of the sun on July 1, 2000. Mars is almost 1 north of the sun. Zoom in to see that it is at full phase - but the superior planets are always near full and can never be seen as crescent. Zoom back out and step forward through time in 1-day intervals. Mars is moving very slowly eastward against the distant stars, but the sun is moving faster and leaves Mars behind. Unlike the inferior planets, which move faster than the sun and reappear east of the sun in the evening sky after their superior conjunction, Mars loses ground and reappears west of the sun in the morning sky. The relative speed of the sun and Mars is very low, and it takes at least a month for Mars to become far enough west of the sun to be visible above the horizon in morning twilight. Because it is on the far side of the sun, it is tiny as seen through a telescope and not especially bright at 2nd magnitude, so its cycle does not begin dramatically. On January 1, 2001, change the time to 4 a.m. Resume running forward through time and notice that each morning Mars is higher at 4 a.m. than the morning before. This is because it rises a few minutes earlier each day. Notice that Mars is a few degrees north of the red star Antares, the rival of Mars, early in March. Around March 15, change the time to 2 a.m. and resume stepping forward through time to notice an interesting change in the motion of Mars. In April, 2001, Mars begins to slow noticeably in its eastward motion against the stars, and on May 15 it stops, is briefly stationary, then reverses its course and begins heading westward in retrograde motion. The planet does not actually move backward, of course; it is an optical illusion, but an effective one. During the several months that the earth, which is traveling faster on an inside path, takes to pass Mars, Mars appears to move backwards against the stars. It is the same effect you get when you pass a slower moving car on the highway - in the time it takes you 141 Planets, Asteroids & Comets to pass it, the other car seems to move backwards in relation to the distant trees and hills, although both of you are moving forward at different rates. Mars moves westward in retrograde motion at an accelerating rate through the summer. The earth passes Mars on June 13, 2001, and then the red planet is at opposition; it lies opposite the sun. Mars rises at sunset and is visible all night long. It is at its closest, 67.3 million kilometers (41.8 million miles) and at its brightest a week later. (The date of opposition and closest approach would coincide if the two orbits were perfectly round, but their noncircularity causes these dates to differ by a few days.). At this time, Mars is about four magnitudes brighter than it was at superior conjunction, or 40 times brighter! Mars and Venus vary in brightness much more than any other planets. You should be able to figure out why this is so. Following opposition, Mars begins to fade as the earth leaves it behind. It sets earlier each day. It continues its westward, retrograde motion until July 19, 2001, and then it seems to stop against the stars again and resume its normal eastward motion. Mars retrogrades toward Antares during early July, but reverses itself before reaching it. On July 1, 2001, change the time to 10 p.m., and resume running forward through time. Zoom in during the late fall of 2001 and notice that Mars is nearly full, but not 100% so. This gibbous shape is as far from being full as an outer planet can be. Mars moves eastward at an accelerating rate, crosses the ecliptic on February 10, 2002, and moves into superior conjunction with the sun again on August 10, 2002. If you want to follow the path of Mars until it is again at superior conjunction, you will have to set the time back by a few hours in March or April 2002. Each time the earth passes Mars, Mars is not equally close because the two planets orbits are not circular. The closest oppo- Section 4 - Our Solar System 142 sitions in the first third of the century come in August, 2003; July, 2018, and September 2035. Telescopically, Mars is a challenging object. Even when closest to earth it is surprisingly - and disappointingly - small, and high magnification on a good telescope with a steady atmosphere are required to see much. Practice is essential too - you will not see much at first glance, even on an ideal night. Allow time for your eye to pick out subtle details. The polar caps are bright and their whiteness contrasts well with the orange deserts that make up most of the planets surface. The giant Hellas basin, when filled with white clouds, can rival the polar caps in brilliance. Indistinct dark markings are visible under ideal conditions and make the Mars aficionado wish for a larger telescope so as to see more. Mars rotates in 24 earth hours and 37.5 earth minutes, so you can watch it rotate significantly during the course of an evening. Each night we see it turned slightly from its orientation at the same time on the previous evening. Open the file MarsZoomed and step forward at one day intervals to watch new features rotate into view while others rotate out. Each night surface features are offset by 37 minutes from the previous night, and during the course of a month you can see the entire surface. You will also notice Mars two moons (Phobos and Deimos) come in and out of view. These moons revolve very rapidly - Phobos takes seven hours to complete an orbit of Mars, while Deimos takes 30 hours. These moons are visible only in very large telescopes. Jupiters path resembles Mars path (and all the other superior planets), although its cycle is shorter. Open the file Jupiter. We begin with it on the far side of the sun on July 19, 2002. Step forward through time at one-day intervals, and change the time of day when necessary in order to keep Jupiter above the horizon. The sun seems to race away, leaving it behind, and Jupiter first appears in the morning sky less than a month later, where its great brightness 143 Planets, Asteroids & Comets (second only to Venus) makes it relatively easy to see. It rises four minutes earlier each day (the same as the stars, since Jupiters motion against the stars is so very slow) and eventually it rises before midnight and moves into the evening sky (October 26, 2002). On December 5, 2002, the earth begins to catch Jupiter, and Jupiter begins its westward retrograde motion. The earth and Jupiter are closest on February 2, 2003, which is also when it is at opposition (to within a few days). Jupiter ends its retrograde motion on April 5, 2003, resumes moving eastward, and is still moving eastward when it disappears into the glare of the sun in August. It is at superior conjunction on August 21, and the 13-month cycle is completed. Jupiters great distance means that its size and brightness change little during its orbit, and it remains impressively large compared to the other planets. Even a small telescope will show a few cloud bands aligned parallel to its equator, and perhaps the Great Red Spot, which is a storm in Jupiters atmosphere. A good telescope will show more features than a person can sketch. The planets rapid 10-hour rotation allows you to see the entire surface over the course of one long night. The file JupiterZoomed shows the planets rotation at 5-minute intervals over a long winter night. Visible in this file are Jupiters four major moons. These moons are a constant delight to amateur astronomers, and Starry Night will show the four major ones, which were discovered by Galileo. They can be seen in any telescope and even in binoculars. They orbit the planet so quickly (1.68 days for Io to 16.7 days for Callisto) that they change appearance night by night. The gas giant Jupiter. The Great Red Spot is visible on the lower right. Section 4 - Our Solar System 144 Their motion can be seen almost minute by minute if they are next to the planet or to each other. Amateurs with decent telescopes enjoy watching the moons pass in front of Jupiter, cast their shadows on Jupiter (the shadows are dark and contrast well against the bright white clouds), disappear in eclipse in the planets shadow, become occulted by the edge of the planet, or even (very rarely) eclipse each other. Jupiters moons eclipse and occult each other when the plane of their orbit is seen edge-on to our line of sight, and this happens in June 2003, April 2009, Nov. 2014, March 2021, and Oct. 2026. Change the date in the JupiterZoomed file to the current date, if Jupiter is visible tonight, to see the changing configuration of its moons. The motions of Saturn and the other outer planets are similar to Jupiters and will not be treated here. You can use Starry Night to follow their motions many centuries into the future. Saturn, however, has rings and moons that set it apart. Saturns rings are one of the spectacles of the night sky when seen through a decent telescope. Several rings orbit the planet, nowhere touching it, and they have different brightnesses and widths. The rings are composed of grains of ice and rock. The planet itself is quite bland, but the rings make up for the lack of clouds. Saturns rings wrap around the planets equator. Saturns pole is tilted relative to its orbital plane, and as the planet orbits the sun we see its rings at a c o n s t a n t l y - c h a n g i n g orientation that follows Saturns 30-year orbital period. When Saturns pole is tilted the greatest amount towards or away from the earth, as it is in 2002 and 2017, we see the rings open to our view with a tilt of 27 The rings of Saturn 145 Planets, Asteroids & Comets degrees; they are wide and bright. Half-way between these dates, the rings are briefly edge-on to our view, and then they almost disappear. They are edge-on in September 2009 and March 2025, but the planet is behind the sun on both these occasions and we will miss the novelty of seeing Saturn without its rings. We will not actually see a ringless Saturn until 2038. Open the file SaturnRingTilt and run forward through time to see how the appearance of Saturns rings will change over the next few decades. Saturns moons are more challenging than Jupiters major four, but huge Titan can be seen in any telescope and even a modest telescope will show one or two others. Ninth-magnitude Titan orbits Saturn in 16 days, and it lies five ring-diameters from Saturn when east or west of the planet. Rhea, Dione, and Tethys are between 10th and 11th magnitude and orbit even more quickly, closer to the planet. Uranus and Neptune are bright e      !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~nough to see in binoculars or small telescopes, and Uranus has been spotted with the naked eye, but the trick is to know where to look. Their tiny greenish disks distinguish them from stars under high magnification (which is how William Herschel discovered Uranus), but they show no surface features and their moons are visible only with large telescopes. They move slowly from constellation to constellation and hold little interest for amateur astronomers. Use Starry Night to see if they are visible tonight. Pluto looks like a 14th magnitude faint star, and it can be spotted only in a very large amateur telescope. Pluto is much fainter than the faintest stars Starry Night Backyard can show, so if you wish to find Pluto with your giant telescope, use Starry Night Backyard to find Pluto's coordinates and then mark its position on a detailed star chart. Starry Night Pro owners can just make their star charts with Pro, as it does include enough stars fainter than Pluto to make detailed charts. Confirm that you have found it when you record its slow movement against the stars from night to night. Section 4 - Our Solar System 146 Conjunctions As the planets orbit the sun and move across the sky, one occasionally passes another, and when this happens it is called a conjunction. Conjunctions happen almost every month and are not rare, but some are more interesting than others. Some are spectacular. A conjunction is defined as the moment when two planets have the same ecliptic longitude (see the section on Co-ordinate Systems in chapter 2.2 for a definition of ecliptic longitude). Alternately, it is the moment when two objects have the same right ascension, which is slightly different. Neither is necessarily the exact moment when they are at their closest. A conjunction looks different from different locations on earth because of parallax. The conjunctions (superior and inferior) which we looked at in the above section all involved the sun and another planet, but the most interesting conjunctions do not involve the sun, but instead two or more planets, or one or more planets and the moon. Mercury and Venus move so quickly that they are involved in most conjunctions that occur. Slow-moving Saturn, in contrast, seldom passes another planet, but is itself often passed. The moon is in conjunction with each planet once a month, and if it comes near the bright planet Venus, Mars, or Jupiter, it can be a spectacular naked-eye sight. You can discover conjunctions to your hearts content by running Starry Night through time. A conjunction is interesting to amateur astronomers when the two objects come close enough to each other to be visible at the same time in a pair of binoculars or, exceptionally rarely, through a telescope. Even rarer is an occultation of one planet by another. None will occur within the lifetime of most people now alive. You will find a sampling of mutual planetary occultations in the files for this chapter; those included took place in 1590, 1771, and 1808. At 12:45 on November 22, 2065 the centers of Venus and Jupiter will 147 Planets, Asteroids & Comets be 16 arcseconds apart and the northern edge of Venus will pass in front of Jupiter! Mark it on your calendar or preview it by opening the file Conjunction 2065. Three or more planets cannot be in conjunction simultaneously, although the word is sometimes misapplied when several are close to each other. Such a grouping of planets is called a massing, and you can discover massings with Starry Night. The best in history was in early 1953 BC, and it was profoundly important to the ancient Chinese, who recorded and remembered it. Open the file Massing 1953 BC and step forward at 1-day intervals through the month of February to see a truly amazing sight develop in the predawn sky! There are files with this chapter for planetary massings on September 29, 2004; December 10, 2006; and May 11, 2011 (Massing 2004, Massing 2006, and Massing 2011 respectively). A popular misconception is that all the planets sometimes line up like billiard balls in a row, and dramatic images of such an alignment are even seen on book covers. The reality is different. Jean Meeus, the foremost thinker about planet alignments, has calculated how close the planets come to being in a line. He finds that, between the years 3100 BC and 2735 AD - a time span which includes all of recorded history - the minimum separation of the five naked-eye planets was 4.3 on February 27, 1953 BC (the massing we just looked at in the file Massing 1953 BC). The best groupings closest to the present are: April 30, 1821 (19.7); Febru- The massing of all five naked eye planets in 1953 BC. Section 4 - Our Solar System 148 ary 5, 1962 (15.8); May 17, 2000 (19.5); and September 8, 2040 (9.3). The 2040 grouping, which includes the crescent moon at no extra charge, will be spectacular (examine it in advance by opening the file Massing 2040). If you consider the three outermost planets too, all the planets never line up. A triple conjunction happens when a planet makes its retrograde loop near a star; the planet passes that star once in its normal forward motion, a second time when in retrograde motion, and a third time when it resumes its forward motion. Each outer planet is in triple conjunction with all the stars within the bounds its retrograde loop, but occasionally a bright star (or even another planet!) is within that loop. (The object does not have to be within the loop itself, but within the limits of the loop in Right Ascension or ecliptic longitude.) One famous triple conjunction is the triple conjunction of Jupiter Star of Bethlehem The famous Star of Bethlehem, seen so often on Christmas cards and topping off Christmas trees, may have been a conjunction of planets. Astrologers of the time looked to the sky for omens, and one of the best was a conjunction of bright planets. In the summers of 3 and 2 BC - the years when Jesus is most likely to have been born - there were two very close conjunctions of Jupiter and Venus (Venus actually occulted Jupiter as seen from South America in the second conjunction) and three conjunctions of Jupiter and the star Regulus. It is plausible that the wise men, who were astrologers, would have interpreted so interesting a series of conjunctions as the fulfillment of ancient prophecies. 149 Planets, Asteroids & Comets and Regulus that is associated with the star of Bethlehem. Open the file ConjunctionChristmas. You begin on August 1, 3 BC. Follow Jupiter and Regulus during the following ten months, adjusting the time of day as necessary to keep Regulus above the horizon. Some people whose knowledge of astronomy is limited are concerned that when planets align, their gravity is somehow magnified and they exert a major pull on the earth. The gravitational or tidal force of a planet is completely insignificant, and even when several are massed together it has no effect on our planet or on ourselves. Planetary alignments do not cause cosmic disasters. Minor Bodies: Asteroids and Comets On New Years Day, 1801 - the first day of the 19th century - the Italian astronomer Giuseppe Piazzi discovered a new planet between the orbits of Mars and Jupiter. It was much smaller and fainter than the other known planets, so it was called a minor planet or asteroid (from its starlike appearance). Since then, over 10,000 asteroids have been discovered and they are a fascinating group of objects. Asteroids are fragments of small planets that were shattered in collisions with each other early in the history of the solar system. Jupiters gravity prevented the formation of one big planet inside its orbit, and the many asteroids are the result. Most asteroids orbit within the asteroid belt - a wide zone between the orbits of Mars and Jupiter - but many others have been knocked out of the asteroid belt during subsequent collisions and wander throughout the solar system. The largest asteroid is Ceres, which has a diameter of 930 kilometers (580 miles). It and all the other large asteroids remain safely within the asteroid belt, but small ones, which can drift throughout the solar system, can strike the earth. When one does, it falls through Section 4 - Our Solar System 150 the atmosphere as a meteor, and if it survives to reach the ground it is called a meteorite. Meteorites are fragments of shattered planets. The impact of a giant asteroid fragment (or comet fragment) is widely credited for the extinction of the dinosaurs 65 million years ago. Most meteorites are stone, but a few are made of iron. The largest asteroids can be seen with binoculars or a small telescope if you know where to look. Starry Night will show asteroids in the Planet List, and you can centre and lock on them or turn on their orbits like any other object. Open the file Asteroids and start time running forward to see the orbits of 5 major asteroids, all of which are between Mars and Jupiter in the asteroid belt. Unlike the orbits of the planets, the orbits of these asteroids (particularly Pallas) are tilted at large angles to the plane of the ecliptic. The thrill in observing asteroids is simply to find and track them. Dozens are within the range of amateur equipment. The planets and minor planets circle the sun with great regularity and we can know exactly where they will be thousands of years in the past or future. This is not true for comets. Most of them have not yet been discovered - and wont be for thousands of years! Comets were born in the outer fringes of the solar system, and that is where nearly all of them still reside. There is a huge reservoir of billions of giant ice balls beyond the orbit of Neptune, of which Pluto is the largest member. If these ice balls stay there, we will not know of them (except by diligent searches conducted with large telescopes) and they will not bother us. However, occasionally one is deflected out of its distant but stable orbit into a new orbit that brings it into the inner part of the solar system, and then we see it as a comet. Comet Hale-Bopp, for example, was first sighted near the orbit of Jupiter in 1995, about two years before it was at its brightest and closest to the sun. A comet is a chunk of ice and dust only a few kilometers across. The ice is mostly frozen water and carbon dioxide and the dust is 151 Planets, Asteroids & Comets simple silicates. When far from the sun, the ice is frozen and the comet is too small and too faint to be seen. But as the comet approaches the sun, it warms. Sunlight thaws the ice, which evaporates (it does not melt - there are no puddles in space!), carrying with it the dust that was embedded in it. The gas and dust form tails that can be long and beautiful. Sunlight causes the gas to fluoresce while the dust merely reflects sunlight, and the gas tail often has a different shape and colour than the dust tail. A comet is brightest when it is closest to the sun, which is when it is warmest, and that is when it is traveling the fastest. If it is near the earth at the same time, it can zip across our sky at the rate of several degrees a day. As it leaves the suns vicinity, it cools, becomes fainter, and slows down. It may not return for hundreds or many thousands of years. Some comets have been captured into short orbits and appear every few years, but these comets have had their ice evaporated after spending so many years near the sun and they are exhausted, and all are faint, with the exception of one - Halleys. Halleys Comet is the only bright comet that returns often enough for each person to have a chance to see it, and many people have. It returns approximately every 76 years, most recently in 1986. Like any other comet, Halleys loses ice through evaporation every time it approaches the sun, and it is not as bright now as it was the first time it passed through the inner solar system. Several prominent comets are shown in the Planets List in Starry Night. Open the file Hale-Bopp and step through time to see the passage of the famous comet near its peak brightness back in 1997. Now open Hale-Bopp 2. Step Starry Nights representation of Comet Hale-Bopp. The gas tail, is quite narrow, while the dust tail is more widely scattered. Section 4 - Our Solar System 152 Meteors and Meteor Showers Most meteors come from comets. Comets shed dust as their ices evaporate, and this dust never returns to the comet. The dust drifts around the sun, following the orbit of the comet. If a dust particle is swept up by the earth, it falls through the atmosphere towards the ground. Friction with air molecules heats it to incandescence and it bursts into flames; we see it as a meteor. Alternate popular terms are falling star or shooting star. If the earth passes through or near the orbit of a comet, we pass through a region filled with dust, and meteors fall by the hundreds. A shower happens at the same time each year, when the earth returns to that part of its orbit. The shower can last for a few hours to a few weeks. Listed below are the best meteor showers, their peak dates, and the number of meteors per hour an observer in a dark location might optimistically see: Shower Date Hourly Rate Quadrantid January 3 85 Eta Aquarid May 5 30 Delta Aquarid July 29 20 Perseid August 12 100 Orionid October 22 20 Geminid December 14 100 Ursid December 22 45 The Eta Aquarid and Orionid showers come from Halleys Comet on the inbound and outbound legs of its orbit respectively. The Geminids come from the asteroid Phaethon. 153 Planets, Asteroids & Comets through time to watch the comet approach the sun and then fly away again. Note how its orbit is very different from the near-circular orbits of the planets. New bright comets are announced on the Internet as they are discovered, and you can learn about them by going to the comets section at www.livesky.com The characteristics of their orbits are published in the same announcements, and if you were to download the orbital elements and follow the directions in Starry Night s users guide on adding a comet, you can add the new comet to Starry Nights database and follow its motion across the sky. Blank Page Section 5 Deep Space ONCE we get out of the solar system, the motion of objects in the heavens becomes a lot less confusing. Stars, galaxies and other deep space objects are so far away from us that their position along the celestial sphere will change little over our lifetime. This sense of permanence allows us to associate patterns of stars as constellations, and describe where a deep space object is with regards to these constellations. Chapter 5.1 describes the various deep space objects, chapter 5.2 gives more background on the constellations, and chapter 5.3 concludes with a tour of the highlights of the constellations, covering many fascinating deep space objects along the way. 5.1 STARS AND GALAXIES Stars We learned in chapter 1.1 that we classify a stars brightness by a number called its magnitude. A stars magnitude tells you how bright that star appears to your eye as seen from earth, but it tells you nothing about its intrinsic brightness (the amount of light it emits). For that reason, magnitude is more properly called apparent magnitude. To know a stars intrinsic brightness, you must know its distance from earth. Obviously, the closer a star is, the brighter it appears, and many of the brightest stars in our sky are among the closest. Sirius, Procyon, Altair, and Alpha Centauri are excellent examples of very close and very bright stars. But if you could line all the stars up side by side, at the same distance from earth, you would see that some outshine others by many thousands of times. The absolute Section 5 - Deep Space 158 Sun Earth in July Nearby star Earth in January Distant stars magnitude compares how bright different stars would appear if they were all the same distance from Earth. As with apparent magnitude, a lower number means a brighter star, and a difference in absolute magnitude of 1 corresponds to a brightness difference of 2-1/2 times. The sun has an absolute magnitude of 4.8, which is about average for stars. Faint dwarf stars have absolute magnitudes as high as 15, Determining the Distance to the Stars We find the distance to the nearest stars by measuring their parallax parallax. Parallax is the shift of a nearby object against more distant objects when seen from two positions. A simple demonstration of parallax is to view an outstretched finger on your hand, first with your left eye and then with your right eye; notice how your finger shifts position relative to the background as your viewpoint shifts from eye to eye. If you move your finger farther away and perform the same experiment, you will notice that the position shift is smaller. The same idea applies to measuring astronomical parallax. Astronomers record a stars position, and then record 159 Stars and Galaxies while bright giant stars like Antares and Deneb have absolute magnitudes as low as -5. To find a stars absolute magnitude in Starry Night, double-click on the star to bring up its Info Window. The stars we can see from earth are not representative of all stars. We see the true powerhouses when we gaze into the night sky, as almost all the stars visible to the naked eye have a lower it again six months later when the earth has moved halfway around the sun and is 300,000,000 km from where it was at the time of the first observation. This will cause the stars position in the sky to shift, and the amount of this shift is the parallax. More distant stars will shift less, therefore having a smaller parallax. The technique works for relatively nearby stars, but distances to distant stars, star clusters and galaxies are determined through indirect means because these objects are so far away that their parallaxes are too small to measure accurately. This means that distances to these objects must be used with caution. You will see wildly different distances quoted for a star cluster, for example, each of which represents the best effort of an astronomical research project. The latest and presumably most correct values are used in this book, but they too are subject to revision and should not be taken literally. Parallax is usually expressed in arcseconds, and this explains the origin of a strange-sounding word. A parsec is the distance an object would have to be if it had a parallax of one arcsecond, and one parsec equals 3.26163 light years. The parsec is the most commonly used unit of distance for astronomers. Section 5 - Deep Space 160 absolute magnitude (are intrinsically brighter) than our sun. The most luminous stars are so energetic that they shine like lighthouses across the gulf of space and can be seen from enormous distances, while a typical star shines brightly only in its own neighborhood and the faintest are hard to see even from nearby. The common types of stars exist in abundance (in fact, our sun is intrisically brighter than most stars), but for the most part are invisible without a telescope. Look closely, and notice that some stars have color. Most stars appear white, but a few have a slight blue or orange hue. Star colors are not saturated, and your eye is not very sensitive to colors in low-light level objects, so star colors are subtle. A reflecting telescope will enhance star colors, although inexpensive refracting telescopes often cause stars to exhibit false colors through incorrectly refracted starlight. Deneb, Rigel, and Spica are bluish, while Antares, Aldebaran, and Betelgeuse are slightly orange in color. The color of a star tells you its temperature, and its temperature tells you what kind of star it is. Red stars are cool and blue stars are hot. Bright red stars (which actually look orange or even yellow) are red giants or supergiants. These giants are hugely swollen stars with bloated atmospheres. They can be so large that, were one placed at the center of our solar system, Mars would orbit beneath its surface! Blue stars shine brightly not because of their size, but because of their intense heat. Their high surface temperature of 25,000 C for a blue giant vs. 3,500 C for a red giant (45,000 F vs. 6,300 F) gives them luminosities of 10,000 times the sun or greater. Other red stars, called red dwarfs, are the most common type of star, but all are so intrinsically faint that none, including the closest, can be seen without a telescope. By playing with the sliders in the Brightness and Contrast option in the Sky menu (the Settings menu in Starry Night Pro), you can have Starry Night draw the stars in exaggerated shades of blue and red. 161 Stars and Galaxies Slightly more than half of the stars you see sit alone in space (disregarding any planets they may have). The rest are accompanied by a companion star or stars, and these are called double stars, triple stars, or even multiple stars. They are very popular targets for amateur astronomers with telescopes of all sizes, as the telescope often reveals two or more stars where the unaided eye sees only one. Most double stars are true binary systems, and the two stars are in orbit around a common center of gravity. The star info window in Starry Night will tell you if a star is a double, and will give the angular separation between the two components. Open the file Mizar to see a famous multiple star. You begin by looking at the star Mizar (in the handle of the Big Dipper) with a regular 100 field of view. Use the Zoom button to zoom in. Once your field of view is down to about 20, you will see that Mizar is actually two stars, Mizar and Alcor. If you let the cursor hover over each star, you will note that their distances are similar, indicating that they are most likely gravitationally bound. Keep zooming in. When you reach a field of view of about 20', you will see Mizar again split into two stars, officially called Mizar A and B. These stars also have similar distances and are part of the same star system. Recent observations have revealed that all three of these stars have faint companions, so there are actually six stars in this system! Occasionally two stars happen to line up as seen from earth, even though one lies far beyond the other, and such pairs are called optical doubles. Open the file AlphaCap and zoom in to see this optical double in Capricorn split into two distinct stars. The brighter one, Algedi is 109 light years away, while the fainter star is almost 700 light years distant! Clearly this is just a chance alignment, not a true multiple star system. Some amateur astronomers enjoy finding and monitoring variable stars - stars that change their brightness. Some variable stars, like Delta Cephei, are regular and predictable, pulsating like Section 5 - Deep Space 162 clockwork in a cycle that takes a few days. Others, like Mira, follow a rough pattern that is similar year-to-year but that can hold surprises. Some variable stars have huge changes in their brightness from maximum to minimum. Mira, for example, goes from a magnitude of about 3.5 at maximum to 9.5 at minimum, which means a change in brightness of a factor of 200! The star info window in Starry Night will tell you if a star is variable. One of the areas where amateurs can make a serious research contribution is by monitoring variable stars, systematically recording their brightnesses, and forwarding the observations to a centralized authority like the American Association of Variable Star Observers (AAVSO). There Naming A Star You cannot name a star for a friend or loved one. Although several companies will offer to name a star and to register it in an official document, for a fee, such vanity registries have no legality. They are simply listings in the companys own book and no one else will recognize the stars name. The responsibility for naming stars, as well as comets, newly-discovered asteroids and moons, surface features on other planets, rests with a committee of the International Astronomical Union. Although they have been busy naming newly revealed craters on Venus, for example, they do not sell star names. Star names sold by private companies have no official status, and these companies actually sell nothing more than pretty pieces of paper at exorbitant prices. Save yourself the expense and print your own equally-valid certificates on your home computer. 163 Stars and Galaxies are so many variable stars that it is impossible for professionals to make all these observations, so they rely heavily on the AAVSO data. Star Names The familiar star names that sound so foreign to our ears are, for the most part, Arabic translations of Latin descriptions. Arabic astronomers translated into their language the Greek descriptions of Ptolemy, and centuries later scribes in the Middle Ages copied and recopied the text, introducing errors while copying words they did not know, until the origins and meanings of some words became difficult to decipher. A few stars names are relatively modern and some were invented as recently as this century. This representative list of common and unusual star names gives a feeling for how stars got their names. Aldebaran: from Arabic for the follower, as it follows the Pleiades Star Cluster across the sky. Algol: from Arabic al-gul - the ghoul; the Arabs apparently noticed - and were bothered by - the way it varies its brightness. Antares: from Greek anti-Aries, meaning against Mars (or more colloquially rival of Mars). Aries is Greek; Mars is Latin. Betelgeuse: a corruption (copying error) of bad from yad aljauza - the hand of al-jauza, the Arabs Central One. Regulus: diminutive of king, (as in regal); named by Copernicus Sirius: from Greek serios for searing or scorching Thuban: corrupted Arabic for serpents head Vega: falling in Arabic, as the Arabs thought of it as a bird falling from the sky Section 5 - Deep Space 164 Fainter naked-eye stars were given numbers or Greek letters according to schemes devised centuries ago by John Flamsteed and Johann Bayer respectively when the first detailed star charts were printed in book form. Starry Night displays common names as well as Bayer letters and/or Flamsteed numbers in the star Info Window. If a star is too dim to have a common name, a Bayer letter, or a Flamsteed number, it is identified in Starry Night by its Hipparcos (HIP) catalog number or its Tycho (TYC) catalog number. These two catalogs were the product of the Hipparcos project, a mission by the European Space Agency to calculate the distances to nearby stars by measuring their parallax (the star distances in Starry Night come from this project). The Hipparcos catalog has about 100 000 stars, and the Tycho catalog has about a million. Many other star catalogs exist, and other texts may refer to a star by its number from one of these catalogs. Star Clusters and Nebulae Anyone who has attended a public star party, where amateur astronomers conduct guided tours of the wonders of the sky with their telescopes, has seen at least a few star clusters and nebulae. They are indeed the showpieces of the night sky. Searching them out and examining them is one of the sublime and endless activities that delights amateur astronomers (and causes them to crave everlarger telescopes). Star clusters come in two varieties: open clusters (sometimes also called galactic clusters) and globular clusters. The two have distinct appearances and histories. Clusters were born together out of enormous clouds of gas, and the stars of a cluster are all the same age. The brightest open star clusters can be seen with the unaided 165 Stars and Galaxies eye. These include the Pleiades and Hyades clusters in Taurus and the constellation Coma Berenices. Several others - the Beehive in Cancer and the Perseus Double Cluster in Perseus - are visible to the unaided eye but are best in binoculars or low-power spotting scopes. (See the descriptions of these objects in the sections on their respective constellations in chapter 5.3). Hundreds more await amateurs with their telescopes. You can search for the most popular ones by name and display them in Starry Night. Open star clusters take their name from their open or undefined shape. They lie in and near the arms of our Milky Way Galaxy. Some are forming still, many are quite young (astronomically speaking), and the oldest were born as much as a billion years ago. The stars in an open cluster are weakly held to each other by gravity, and the cluster eventually evaporates by losing its outermost stars which escape into interstellar space. More than 1,000 are known, and they contain from a dozen to a few thousand stars. Each has its own characteristics, some have unusual appearances (like the Pleiades, which resembles a very little dipper), and no two look entirely alike. Globular star clusters, in contrast, were born during the early days of the Milky Way and are as old as our galaxy itself. They are huge compact spherical balls of ten thousand to a million old stars, and at first glance they all look identical. A closer examination shows that they have varying degrees of compactness, but differences between them are indeed subtle. About 100 are known in our galaxy. They form a halo distributed around the center of the Milky Way, and they avoid the spiral arms. Because they pass near the center of The Pleiades, an open star cluster in the constellation Taurus. Section 5 - Deep Space 166 the Milky Way at some point in their orbit, they appear concentrated towards that direction, which is in Sagittarius in the summer sky. None are very close to our solar system. Whereas many open star clusters lie a few hundred to a thousand light years from earth, a typical globular cluster is tens of thousands of light years distant, and its stars are proportionately fainter. Only a small handful (such as M13 in Hercules and M22 in Sagittarius in the Northern Hemisphere and Omega Centauri in the Southern) are visible to the unaided eye, and they look like faint stars. Only through a telescope can their structure be seen. Open the file M13 and then zoom in to see the structure of the Hercules Cluster. A nebula is an enormous cloud of gas (nebula is Latin for cloud). Our Milky Way is permeated with gas, most of it hydrogen and helium, which is concentrated in its spiral arms. For reasons not completely understood, this gas can become concentrated into comparatively dense nebulae. If they are illuminated by nearby stars, nebulae shine brightly and are beautiful tenuous wispy clouds when seen through a telescope; if unilluminated, they appear as blotches of darkness silhouetted against the myriad stars of the Milky Way. The brightest, the Orion Nebula, is barely visible to the unaided eye as a fuzzy star in Orions sword. It is impressive through any telescope if the sky is dark (being low-contrast objects, nebulae require a dark sky to be seen well). Several others that are pretty when observed with amateur equipment lie in Sagittarius. The most famous dark nebula is the Coal Sack in the Southern Cross. Nebulae are the birth-places of stars, and several are giving birth to brand- M16, the Eagle Nebula 167 Stars and Galaxies new clusters of stars right now. Hot newborn stars then disperse the remaining nebulosity. When the Milky Way is much older than now, its gas will be exhausted and no new nebulae - and no new stars - will form. Another type of nebula, called a planetary nebula, is entirely different. A planetary nebula is a shell of gas expelled by a aged or dying star. The Dumbbell Nebula in Vulpecula and the Ring Nebula in Lyra are pretty sights in amateur telescopes and are popular showpieces at summer star parties, but most are faint and a challenge to find. Their confusing name comes from their superficial resemblance to the planets Uranus and Neptune as seen through a small telescope. Open the file Orion Nebula to see a true nebula, and then open Dumbbell to see a planetary nebula. Messier Objects You have probably noticed that many of the objects we have been looking at have the letter M followed by a number. These are objects in the Messier catalog. There are 110 objects in this catalog, which was compiled by astronomer Charles Messier in the 18th century. These objects are either star clusters, nebulae, or galaxies. Messier was a comet hunter, and his goal in making this catalog was to identify fuzzy objects which were commonly mistaken for comets! Today the catalog is more frequently used as a guide to some of the most beautiful objects in the sky. Because Messier was based in France, the catalog has a definite Northern Hemisphere bias, and many treasures in the Southern sky are missing from his catalog. Starry Night includes full-color images of all the Messier objects. Section 5 - Deep Space 168 The Milky Way Everything we have looked at so far in this book is inside our own galaxy, the Milky Way. Much as the two-dimensional plane of the ecliptic appears as a one-dimensional line when viewed from earth, the threedimensional Milky Way appears as a twodimensional band of countless stars that wraps around the sky. This band appears brighter than the surrounding area, and its brightness comes from the combined light of billions of dim stars. The Milky Way is the largest and grandest structure in the sky. Open the file Milky Way to see the outline of the Milky Way across the southern part of the celestial sphere. (If you want to change the color of this outline to make it more prominent, choose Sky | Sky Settings (Settings | Milky Way in Starry Night Pro), then click on the color bar beside the words Milky Way Color to select another color.) Now choose Sky | Milky Way to turn off this outline. You can still see that the area of the sky inside the Milky Way has many more stars than the surrounding regions. Turn the Milky Way outline back on and scroll around to see what other constellations the Milky Way passes through. You can see that the Milky Way is not uniformly wide and bright. It is much brighter and wider in the portion that extends from Sagittarius south to the Southern Cross, and it is comparatively thin in the opposite direction, towards Perseus and Auriga. This asymmetry shows us the direction to its center, which lies about 28,000 light years from us in the direction of Sagittarius. A view of the Milky Way from Starry Night. 169 Stars and Galaxies The Milky Way has a third dimension: depth. Because we are inside the Milky Way, we cannot view its entire extent. However, by mapping the distribution of stars and gas, scientists know that it would look like a giant flat pinwheel with a bright center and spiral arms made of stars and clouds of gas, all of which is surrounded by a halo of dispersed stars and unknown material. This structure is known as a spiral galaxy. Open the file M100 to see another spiral galaxy, one that looks very much like our Milky Way would from the outside. The Milky Way is over 100,000 light years in diameter but only a thousand light years thick. It contains between a hundred billion and a trillion stars. The very center of the Milky Way seems to be inhabited by a giant black hole with the equivalent mass of 2.6 million suns. The black hole is surrounded by spiraling clouds of hot hydrogen gas. None of this can be seen by our eyes because of numerous dust clouds which lie between us and the center, but it has been detected and mapped by radio astronomers. Surrounding the Milky Ways core is the central bulge which is several thousand light years in diameter and which contains billions of stars. You can see part of the central bulge on a summer evening when you look at the Great Sagittarius Star Cloud. Radiating from the center are a suite of spiral arms. No one knows what causes the arms to form or what maintains their structure. They are bright because they contain gas and the hot young stars that were recently born out of the gas. The Small Sagittarius Star Cloud is a small portion of the innermost arm, which is about 6,000 light years distant. The spiral arms rotate around the center. At our distance from the galaxys center, the sun takes 230 million years to make one revolution - a period of time called a galactic year. We are moving towards Cygnus as we orbit the Milky Way. Section 5 - Deep Space 170 Our sun lies on the inside edge of the Orion Arm. When we face Orion, we are looking at the nearest arm to us, which is part of the reason there are so many bright stars in Orions vicinity. The Orion Nebula is near the arms center. Beyond lies the Perseus Arm at a distance of about 7,000 light years; the Perseus Double Cluster is within it. Beyond lies the sparse outer edge of the Milky Way. All the stars that make up the constellations inhabit a tiny portion of the Milky Way. We see just our neighborhood. The naked-eye stars are less than 2,000 light years from earth (most are much closer), and this is less than 2% of the diameter of the Milky Way. Binoculars (or a low-power wide-angle spotting scope) are the best tool for exploring the Milky Way. Take the time to explore it at leisure when you are away from city lights. As much as weve learned about the Milky Way, serious questions remain unanswered. We do not know how it formed, how it acquired its globular clusters, the exact nature or history of the giant black hole at its heart, its precise shape (it seems to have a bar structure running through its center - the Milky Way Bar), how many spiral arms it has, or the nature of the missing mass that is important in holding it all together. Much remains to be discovered before we can claim to know it well. Other Galaxies The Milky Way is our galaxy, but billions of other galaxies exist. You can see three of them with your eyes alone and hundreds with a decent amateur telescope Early this century galaxies were called island universes, which evokes their size and isolation. Galaxies come in a variety of shapes and sizes, but they have in common that they are vast systems of 171 Stars and Galaxies many billions of stars, each with its own history. Galaxies are the fundamental building blocks of the universe, and they stretch away as far as modern telescopes can see - to distances exceeding 10 billion light years. Billions of galaxies are known. The closest large galaxy to the Milky Way is the great galaxy in Andromeda or more concisely the Andromeda Galaxy. It is also the brightest galaxy visible from the Northern Hemisphere. It can barely be seen with the unaided eye on a dark autumn night, but it shows up well even in a small telescope. It lies about 3 million light years from earth and is comparable to the Milky Way in size. (See the section on Andromeda in chapter 5.3). Much closer but visible only from the Southern Hemisphere are the two Magellanic Clouds. These comparatively small galaxies are satellites of the Milky Way, and they orbit around it like galactic moons. They will probably collide with the Milky Way in the distant future and become absorbed into it, as has certainly happened to other small galaxies that approached too close to the Milky Way in the distant past. To the eye, they look like detached portions of the Milky Way. They lie a bit less than 200,000 light years from earth. Beyond are countless galaxies visible only with a telescope. They look like faint smudges of light, but each has its own character and its own orientation. The largest have either a spiral shape like our Milky Way, some with a bar running through their center, or they are elliptical super-huge balls of stars with little if any gas. A few are irregular in shape. The most common are small elliptical galaxies that resemble globular star clusters. It can be hard to distinguish a small elliptical galaxy from a large globular cluster. Open the file Elliptical to see M49, a good example of an elliptical galaxy. The greatest concentration of nearby galaxies lies in the direction of Virgo. Our Milky Way is actually on the outskirts of Section 5 - Deep Space 172 this huge cluster of galaxies whose center is 60 million light years from home. Two dozen Virgo galaxies are bright enough to see in large binoculars. Many amateurs enjoy hunting down galaxies, trying to tease out their structures, and comparing them to each other. Two Messier objects which are close together in the Virgo Cluster. M87 (left) is an elliptical galaxy, while M91 is a spiral. 5.2 ABOUT THE CONSTELLATIONS Introduction In this chapter we return to learn more about the constellations, the fundamental divisions of the sky. Few constellations look like the animal or person they are named after, and you should not be frustrated if you cannot see a princess, bear, or winged horse in the sky. Only in a few cases can the stars be connected to be made to look like a man, a lion, or a swan. Many constellations were named in honor of heroes, beasts, and objects of interest to the people who named them, and not because of any physical resemblance. We do similar things today: the state of Washington looks nothing like the gentleman on the U. S. dollar bill, nor does the bridge in New York City. The exceptions are few, among Section 5 - Deep Space 174 them Orion, Scorpius, Gemini, and Taurus. During the Renaissance, when star maps were designed to be beautiful as well as useful, they were filled with elaborate and colorful drawings that often ignored the background stars. Starry Night will show constellation figures in the classical style if you choose, but notice how poorly they fit the stars. Choose Constellations | AutoIdentify and scroll around to see these classical drawings for each constellation. We learned in the exercise for chapter 1.1 that there is more than one way of drawing the connect-the-dot stick figures which we make to identify the constellations. We looked at the two figure sets available in Starry Night: a common astronomical drawing set and the figures invented by H.A. Rey. His popular book The Stars: A New Way to See Them remains in print five decades after it was first published. It appeals to people who feel the need to find a oneto- one correspondence between stars and figures regardless of whether or not a meaningful fit can be made. In any case, there are no official or correct ways to connect the stars, and you are free to invent your own designs if you like. Stick figures of the same constellation can vary dramatically. These drawings show the constellation Phoenix in both the standard figure (left) and that invented by H.A. Rey. 175 About the Constellations Asterisms Some common star patterns are not actual constellations. These asterisms (from aster, the Greek word for star) can be part of one constellation or parts of two or more constellations. These are a few of the major asterisms; select Constellations | Asterisms in Starry Night (Guides | Constellations | Asterisms in Starry Night Pro) to display them all. Big Dipper: The seven brightest stars of Ursa Major. Great Square of Pegasus: The three brightest stars of Pegasus and the westernmost bright star of Andromeda. Keystone Keystone: Four medium-bright stars in Hercules that form a square Little Dipper: The seven brightest stars in Ursa Minor. Sickle: Stars in Leo in the shape of a harvesting sickle (or a backwards question mark). Summer Triangle: The stars Vega in Lyra, Deneb in Cygnus, and Altair in Aquila. Constellations have both a formal Latin name (Aquarius, for example) and a common English equivalent (Water Carrier). This differs from star names, which are mostly Arabic, as you learned in chapter 5.1. Some constellations pre-date the Latin-speaking Romans, but Latin was the language of scholarship until recently and the Latin names were standardized long ago. New constellations invented in modern times (such as Leo Minor) were given Latin names to conform with ancient custom. Choose Constellations | Constellation Settings | Labels (Guides | Constellations | Constellation Settings | Labels in Starry Night Pro) Section 5 - Deep Space 176 to choose which style of name (or both) you want displayed in Starry Night. See Appendix A for a list of constellations and their abbreviations. At any given moment you can see half the sky - and approximately half the constellations. As the hours pass and the sky rotates overhead, constellations in the west set and are replaced by others that rise in the east. During the course of a long winter night you can see perhaps two-thirds of the sky, and during the course of a year even more. But - unless you live on the equator - some constellations remain permanently hidden from view. The far southern constellations are a mystery to us in the Northern Hemisphere, and their names - Dorado, Tucana, Centaurus - sound romantic. Just as the constellations that lie far to the south remain out of sight (assuming you live at a mid-northern latitude), the constellations far to the north remain visible all year. They do not change with the seasons, although they are more easily visible at one time of year than another. Because they are always available, they seem to have less value and we take them more for granted. The Big Dipper is above North America for much of the year and has little novelty - yet Australians never see it. The most popular and best-known constellations are the 12 that make up the classical zodiac. This is true even though half of the zodiacal constellations contain few, if any, bright stars and are not conspicuous, and several cannot be seen from urban areas. They are well-known because of their astrological associations, but their fame does not correspond to their visibility. It is ironic that everyone has heard of Cancer, although few could find it, while comparatively few people have heard of Auriga, which is a magnificent constellation of bright stars that is filled with star clusters. Just as some people are famous because of where they are (the position they have in our society) rather than who they are, so it is with the 177 About the Constellations constellations. A constellations popularity tells you nothing about how interesting it is. The zodiac is treated fully in Chapter 3.3. History of the Constellations Humans have the need and the skills to find patterns in nature. Our brains are programmed to invent patterns and to impose order on disorder. This talent helped our remote ancestors find their way around hunting and scavenging sites, and it helps us find our way around the sky. We also have a need to feel connected to the cosmos. Being completely inaccessible, the sky is an endless source of mystery and wonder. It takes a soulless person indeed to gaze up at the sky on a dark, starry night and not wonder how we fit into it all. People have been doing just that since the beginning of time. The origins of most constellations are lost in the mists of antiquity, and some of them are prehistoric. We can only unravel as much as we can of the origins and history of the oldest constellations by using the sparse clues available to us. Others were created in more recent times and their history is documented, and the far southern constellations were outlined and named during the Age of Exploration only several centuries ago. As recently as early last century astronomers were proposing new ones, but their number was fixed at 88 early this century and the days of creating new constellations are over. The core of our familiar western constellations probably originated in prehistoric Sumeria. The Sumerians, who lived in the arid land between the Tigris and Euphrates Rivers in what is now southern Iraq, developed one of the worlds first great civilizations and counted the invention of writing among their achievements. They were a superstitious folk, and they paid great attention to corres- Section 5 - Deep Space 178 pondences between events in the sky and events on earth. They saw the seasons as a cyclic battle between the Lion and Bull. In midwinter 6,000 years ago, Leo the Lion stood high overhead while Taurus the Bull lay dying on the western horizon, and the lion was triumphant. The reappearance of the Bull in the morning sky marked the return of spring and the death of winter, and the Bulls turn to triumph. They charted the stars to try to figure out what was The Oldest Constellation? The Great Bear (the brightest part of which is the Big Dipper) is probably the oldest constellation, and it dates to prehistoric times. Bears were worshipped in cave man days in Europe, before they became extinct (in Europe) at the end of the last Ice Age. Bears are still worshipped by nomadic people in Siberia. People named the celestial Great Bear for its behavior. It does not look like a bear, but it does act like a bear. Earth bears hibernate, and so does the celestial bear. Its low in the north in winter and returns in spring in a way that reminded people of bears seasonal behavior. What is truly interesting is that widely separated people around the world - from Europe to Asia to North America - saw these stars as a bear. We can only speculate, but its likely that the concept of the Great Bear originated during the Ice Age and was carried from Europe to Siberia - or the other way around - and then to North America more than ten thousand years ago. If so, this association is one of the worlds oldest surviving cultural artifacts. 179 About the Constellations happening to them and to the world around them, and their ideas of interpreting omens were elaborated upon by the Babylonians, who lived in the same area much later. The Babylonians left the first constellation lists on clay tablets, and they invented the idea of the zodiac around the 6th century BC. Little Babylonian and less Sumerian constellation lore remains today, and that which does is imbedded in the classical Greek stories. The Greeks borrowed the concept of the zodiac from the Babylonians and incorporated Babylonian star stories into their own myths. Later, the Romans borrowed the Greek stories and weve borrowed those of the Romans, so there is a tradition of borrowing and elaborating that dates back at least 6,000 years. We know Greek mythology in detail, but less about how they divided the sky. There are only scattered references to stars and star patterns from early Greek times. The astronomer Ptolemy, who lived in Alexandria around 150 BC, described the 48 Ptolemaic or classical constellations, which remained essentially unmodified for 14 centuries. Perhaps 30 of these are Babylonian in pedigree and the rest indigenous Greek. His main source was a long poem based on an earlier and now lost work from about 350 BC by Eudoxus - the Greek astronomer who constructed the first recorded celestial globe and who worked out the idea of celestial coordinates. Ptolemys book became known as the Almagest (the Greatest) when translated into Arabic. The Arabs gave most of the stars their familiar common names, which are usually Arabic translations of the stars positions as described by Ptolemy. Rigel, for example, comes from Arabic for foot, which is exactly where the star is within Orion. Following the Dark Ages, the Almagest was translated into Latin, the universal language of the Christian world, and reintroduced into Europe around the year 1,000 after an absence of nearly a millennium. The 48 Ptolemaic star patterns form the core of the constellations of the northern sky. Section 5 - Deep Space 180 Many additional constellations were added during the Age of Exploration. When European navigators first ventured into southern waters in the late 1500s and early 1600s, they discovered an uncharted sky in addition to uncharted lands. They divided the southern sky into groups of stars, naming the new constellations after exotic things they found in the new world, like Pavo the peacock and Indus the American Indian. Many of these new constellations achieved legitimacy and permanence by virtue of being included in Bayers great Uranometria star atlas of 1603. (Bayer also introduced the idea of using Greek letters to name the brighter stars in this atlas.) Seven additional constellations appeared in 1690 in a star atlas by the Polish astronomer Johannes Hevelius. He felt that some areas of the sky were too empty and that his atlas would look more attractive if these areas of faint stars only were filled in. His new constellations include Lacerta the Lizard and Vulpecula the Fox. Hevelius is remembered today as the last astronomer to reject using the newfangled telescope and to observe by eye instead. The last burst of constellation-naming is courtesy of the French astronomer Nicolas Louis de Lacaille, who lived in Cape Town, South Africa, from 1750-1752. He created 14 new constellations Microscopium(left) and Telescopium, two of the Southern constellations named after technological objects. 181 About the Constellations from the southern stars, naming them after practical objects like an air pump, a chisel, a microscope, and a pendulum clock. This may not seem romantic to us, but apparently these were fascinating objects in their time, and now they too are immortalized in the sky. They contrast so greatly with the mythological beasts and heroes of the classical age that Lacaille has been accused of turning the sky into someones attic. Through the 18th century, there was no universally approved list of constellations and no official constellation boundaries. Mapmakers were free to add new constellations if they wished and to decide where one constellation ended and another began. This situation was intolerable to astronomers, who were creating ever- Defunct Constellations Not all constellations are ancient. Although the last wave of constellation-inventing ended when the southern sky was finally filled in the middle of the 18th century, astronomers felt free to create their own as recently as the late 1800s. Often constellations were created for political purposes - to flatter a patron, for example - but such contrivances were seldom accepted graciously by other astronomers, and most disappeared as quickly as they appeared. They now are minor footnotes in the history of the sky. Examples include Robur Carolinium, or Charles Oak, invented by Edmond Halley to honor King Charles II (who once escaped death by hiding in an oak tree); Fredericks Glory, a sword that honored Prussias Frederick the Great; and Telescopium Herschelli or Herschels Telescope. Section 5 - Deep Space 182 more detailed star charts, and an early task of the new International Astronomical Union was to settle the constellation boundary ques- The Official Cygnus Cygnus may look like a cross or even a swan, but the actual constellation includes many fainter stars that lie outside the popular stick figure. Since 1928 a constellation has been defined in the same way that a parcel of property is described on earth - by specifying its boundaries as a series of interconnected straight lines. The true definition of Cygnus is everything in the sky that lies within these boundaries (the middle two-thirds of the description is omitted): Mridien de 19 h. 15 m. 30 s. de 27 30' 30 0' Parallle de 30 0' de 19 h. 15 m. 30 s. 19 h. 21 m. 30 s. Mridien de 19 h. 21 m. 30 s. de 30 0' 36 30' m Parallle de 36 30' de 19 h. 21 m. 30 s. 19h. 24 m. Mridien de 19 h. 24 m. de 36 30' 43 Parallle de 28 0' de 21 h. 44 m. 21 h. 25 m. Parallle suite de 28 0' de 21 h. 25 m. 20 h. 55 m. Mridien de 20 h. 55 m. de 28 0' 29 0' Parallle de 29 0' de 20 h. 55 m. 19 h. 40 m. Mridien de 19 h. 40 m. de 29 0' 27 30' Parallle de 27 30' de 19 h. 40 m. 19 h. 15 m. 30 s. (Reference: Dlimitation Scientifique des Constellations, E. Delporte, Cambridge, 1930.) 183 About the Constellations tion once and for all. A commission was appointed to draw up a list of official constellations and to define their boundaries, and the commissions results were approved in 1928. The boundaries are a series of straight lines in the sky that read like legal property boundaries on earth. Starry Night will show the constellation boundaries and names. Since 1928, no new constellations have been invented. Constellations of Other Cultures Our familiar constellations are a product of the history of our western culture. Today, the 88 constellations we know and love are universally recognized. Like our Gregorian calendar, they are official around the world. But it was not always so, and each culture invented its own way of dividing the sky. Sadly, the indigenous constellations of most cultures have been lost and only in scattered remote areas are pre-western constellations still remembered. The ancient Egyptians saw a Crocodile, Hippopotamus, the front leg of a bull (our Big Dipper), and the god Osiris (our Orion). Their Isis is our Sirius - an important goddess whose reappearance was used to predict the annual flood of the Nile. The original Egyptian constellations were replaced by the familiar Greek constellations after Egypt was conquered by Alexander the Great in the third century BC, and in the following centuries their ancient knowledge was almost completely lost. The only clues that remain of ancient Egyptian sky lore come from enigmatic tomb paintings. The Chinese divided the ecliptic into 28 lunar mansions, somewhat analogous to our zodiac (which is solar), and the stars into almost 300 groupings that are smaller than our constellations and that we would call asterisms. Some patterns that look so obvi- Section 5 - Deep Space 184 ous to us that we have a hard time seeing them any other way, like the W of Cassiopeia, are subdivided by the Chinese (in the case of Cassiopeia, into three). They have four seasonal super-constellations, each made of several asterisms with a similar theme: the White Tiger of autumn, the Black Tortoise of winter, the Blue Dragon of spring, and the Red Bird of summer. The Chinese paid less attention to star brightnesses than we do when dividing the sky and they incorporated fainter stars. We are amazed that Lynx is an official constellation, so faint are its stars, but the Chinese regularly included such faint stars in their asterisms. Among the Chinese constellations are the Dogs, the Awakening Serpent, the Wagging Tongue, the Tortoise, the Army of Yu-Lin, the Weaver Maid, and the Cow Herder. Peruvian Dark Constellations The Inca of pre-conquest Peru recognized what we may call dark constellations. At their southern latitude, the center of the Milky Way passes straight overhead and is spectacular. They recognized patterns of bright stars like we do, but they also named the dark dust clouds of the Milky Way. They thought of the dark clouds as earth carried to the sky by the celestial river. Among the dark clouds were Ya-cana the Llama and her baby Oon-yalla-macha, the fox A-toq, the bird Yutu, and Hanp- tu the Toad. The Indians watched for changes in the visibility of the dark clouds, caused by upper-atmospheric moisture, that would tell them whether the coming year would be wet or dry. 185 About the Constellations The Changing Constellations Remember that constellations are areas of the sky, and they have infinite depth, so the stars in a constellation do not necessarily have anything to do with each other, although they appear to lie in approximately the same direction as seen from earth. Two stars that appear to be very close to each other may actually be separated by enormous distances, one far beyond the other, while stars on opposite sides of the sky may be relatively close to each other with us in the middle. You cannot tell just by looking. This third dimension of depth means that the appearance of the constellations depends on your vantage point. Distances within our solar system are so small that the constellations look exactly the same from Mars, Venus, and Pluto (you can verify this with Starry Night). But if we move far beyond our solar system, the story changes. If our earth orbited a distant star rather than our sun, the stars would be distributed in completely different patterns and our familiar constellations would not exist. If aliens live on other planets in orbit around other stars, they have their own constellations. To see this for yourself, open the file LDEarth to view the stars in the Little Dipper as seen from Earth. Now open the file LDAlpha to view the Dipper as seen from Alpha Centauri, our nearest neighbour (about 4 light years away). Arrange the windows so that you can see both drawings of the constellations at the same time. You can see that the bowl of the Dipper has become distorted as viewed from Alpha Centauri. This distortion will increase as we move farther from our solar system, and the seven stars in the Little Dipper will make a pattern which doesnt resemble a dipper at all. Open LDArct to see this. You are now viewing the Little Dipper from Arcturus, which is about 40 light years away from Earth. Clearly the Arcturians would have a quite different name for this star Section 5 - Deep Space 186 grouping than the Little Dipper! They would probably not even group the same seven stars together. Clearly, our constellations are valid only for our solar system. Interstellar Exploration In addition to all the other adjustments that come with moving into a new home, humans of the future who colonize planets around other stars will have the task of dividing their sky into their own constellations. It will be a lot of fun. One of the nearest stars to Earth which we may visit is Barnards Star, about six light years distant. Open the file Barnard to see that the new immigrants would see a bright 0th magnitude star near the belt of Orion which is not in our night sky - because it is our own sun! The stick figure of Orion, as seen from our Earth (right) and from Sirius. 187 About the Constellations Even when viewed from earth, the constellations are not forever. Each star in the sky is actually in motion. Although these speeds are quite high (most stars at moving at several hundred kilometres per second!), the vast distance to the stars mean that this motion is imperceptible to us. Each stars speed and direction is different from its neighbors, causing each star to move relative to its neighbors as seen from earth, over very long periods of time. This very slow motion of a star across the celestial sphere is called its proper motion (proper meaning belonging to, rather than correct). Each stars info window in Starry Night shows that stars proper motion in terms of the amount by which a stars RA and Dec will change each year. (Remember that a stars RA and Dec will also change due to precession, but precession changes the positions of all stars equally.) Our familiar constellations look much the same now as they did at the end of the last Ice Age, but in the distant future they will become distorted by their stars motions and eventually they will become unrecognizable. The constellations of 1 million AD will bear no resemblance whatsoever to the star patterns we know and love today. Eventually people will have to invent new constellations. It would be interesting to know how long people will retain the classical constellations as they become increasingly distorted before revising the scheme and starting over. Starry Night lets you cover a time period of about 15,000 years, but even in this span of time the stars do not move much. However, we can see some movement in the nearest stars, which appear to move faster because they are closer. Open the files LyraPast and Lyra Future and arrange the windows so you can see Lyra as it looked in 4713 BC, and as it will look in 9999 AD. The stars which form Lyra hardly change at all, but we can see that the bright star Vega has moved relative to the other stars in Lyra. Vega has shifted its position more than the other stars because it is much closer to earth. See if you can spot other differences between the two images! The Big Dipper 100,000 Years from Now The Big Dipper really does look like a dipper, but it wont always. The middle five stars are traveling together through space on nearly parallel paths as they orbit around the center of the Milky Way, and they will retain their relative spacing far into the future. The Dippers two end stars, however, are traveling in the opposite direction. Eventually the Big Dipper will become stretched into what people may one day call the Big Lounge Chair. 5.3 HIGHLIGHTS OF THE CONSTELLATIONS Starry Night shows all 88 constellations in the sky. Many constellations - especially the newest ones such as Lynx - are obscure and are of little interest. They have no bright stars and it can be a challenge simply to identify them, even on a dark night away from city lights. Some are eternally below your southern horizon (or northern horizon, if you live in the Southern Hemisphere) and permanently out of sight. But the major constellations are signposts in the sky and they contain well-known bright stars, star clusters, and nebulae that are within the reach of binoculars or small telescopes. A few minor constellations also contain objects of special interest. See Chapter 5.1 for descriptions of the different kinds of star clusters, nebulae, galaxies, double stars, and variable stars. This chapter describes a sampling of bright stars and wellknown deep-space objects within selected constellations. The constellations are organized according to the Northern Hemisphere Section 5 - Deep Space 190 season when they are best viewed during the evening. See Appendix A for a list of all 88 constellations. There is a file for each constellation described in this chapter. To view the files for this chapter in the best manner, when you open Starry Night you should do the following: In Starry Night Backyard, select Constellations | Constellation Settings. Click the Auto Identify button and make sure that the Boundary and Stick Figure options (and no other options) are checked. In Starry Night Pro, double-click on the Constellation Tool and make sure that the Boundary and Stick Figure options (and no other options) are checked This will ensure that the constellation files that you open show the boundary and stick figure of the relevant constellation. Winter Orion (the Hunter) dominates the winter sky. Three bright stars in a row form his belt, two stars mark his shoulders, and two stars mark his knees or feet. A sword hangs from his belt. The two brightest stars in Orion are Betelgeuse and Rigel. They lie at opposite ends of Orion - Betelgeuse is in his left shoulder and Rigel in his right foot - and they have contrasting colors. Betelgeuse is known for its orangeness, while Rigel is the bluest bright star in the sky. Rigel is also a pretty double star. Betelgeuse is enormous. If our sun were placed at its center, the planets out to and including Mars would orbit within it. Such supergiant stars are unstable, and Betelgeuse varies its size and its 191 Winter Constellations brightness slightly and irregularly as it expels material into space. It is 10,000 times as luminous as our sun and lies at a distance of about 520 light years. Betelgeuse is well on the road to exploding as a supernova. Rigel is almost twice as distant at 900 light years, and is also more luminous (50,000 suns). It is approximately the same size as Betelgeuse. It is a blue supergiant star with a surface temperature twice that of our sun. It has a 7th-magnitude companion star 9 arcseconds distant (their true separation is about 50 times the distance from Pluto to the sun). Mintaka (also known as Delta Orionis) is the westernmost of the three bright stars that form Orions belt. It is one of the prettiest double stars in the sky for a small telescope. The main star is magnitude 2.2, and its 5.8-magnitude companion lies 53 arcseconds distant. At a distance of 1600 light years their true separation is about 1/2 light year, or 30,000 times the distance between the earth and sun. The most famous object in Orion is the famous Orion Nebula (M42) - a true showpiece in a telescope of any size. It is the fuzzy naked-eye star that marks the jewel in Orions sword. A telescope reveals it to be a glowing cloud of hot hydrogen gas, illuminated by a group of hot stars (the Trapezium) at its center that are causing the nebula to fluoresce. The visible part of the nebula is 30 light years across and larger in apparent extent than the full moon. It lies about 1600 light years from earth. A detached portion to the south (separated by a dark lane of dust that lies in the foreground) is called M43. The Orion Nebulas outer parts extend over 5 from M42 and cover much of Orion. Open the file Orion to see Orion with major stars labeled and the Orion Nebula. The Orion Nebula Section 5 - Deep Space 192 Canis Major (the Large Dog) follows Orion, the Hunter, across the winter sky. Sirius, the brightest star in the sky beyond the sun, is nicknamed the Dog Star because it is the brightest star in the Large Dog. Sirius is 8.6 light years (95 trillion kilometers or 50 trillion miles) from earth. It is a hot blue-white star twice the diameter and 23 times the luminosity of the sun. Ancient Egyptians noticed that Sirius rose just before the sun as the Nile began to flood, and they began their year with its appearance. Its famous companion star, Sirius B, visible only in large telescopes, was the first white dwarf star known; it is a dead star not much larger than the earth but with as much material as our sun, and is so dense that a tablespoon of its material would weigh a ton. Only 4 south of Sirius lies the bright open star cluster M41. Its 80 stars, the brightest of which are 7th magnitude, fit within a circle 2/3 in diameter. M41 is 25 light years in diameter and 2,000 light years distant. Because of its large size, it is prettiest in binoculars or a low-power eyepiece. Open the file Canis Major to see Canis Major, Sirius, and M41. Taurus (the Bull) is an ancient constellation that dates back to when bulls were worshipped in the Middle East. We see it as a face with a red eye and two menacing horns. Aldebaran - the eye of the bull - is an orange giant star about 150 times as luminous as our sun. It is one of three bright orange stars in the sky, the other two being Betelgeuse in Orion and Antares in Scorpius. It appears amid the stars of the Hyades Star Cluster, but with a 193 Winter Constellations distance of 68 light years from earth it is actually well in front of and only half as distant as the clusters stars. The face of the bull is outlined by the remarkable Hyades Star Cluster - the closest bright star cluster to earth. Recent measurements reveal that its center is 151 light years from earth and it has a diameter of 15 light years. A handful of its stars are visible to the unaided eye, dozens can be seen with binoculars, but the clusters membership totals 200 known stars. The part we see by eye is 5-1/ 2 in diameter, which corresponds to 15 light years, but this is only its core; fainter stars lie up to 12 (40 light years) from its center. This is an old star cluster with an estimated age of 700 million years, and many of its stars are yellow - in contrast to the youthful Pleiades with its blue stars. The prettiest star cluster in the entire sky is certainly the Pleiades, also known as the Seven Sisters. The Pleiades rises an hour before the main part of Taurus, and in Ptolemys time it was considered a separate constellation. A minor mystery is why it should be called the Seven Sisters when only six stars are brighter than the rest, but probably the facts were ignored to fit the mythology of the sisters, who were seven daughters of Atlas. Some people can see 10 or 11 stars, binoculars reveal dozens, and the total count is probably over 500. These are young stars born only about 100 million years ago, and the cluster is not much older than the Rocky Mountains. Blue stars, which burn out relatively quickly, remain, and they illuminate with bluish light a dust cloud that is passing by (visible only in long photo exposures). The Pleiades is 380 light years from earth and about 12 light years in diameter. Open the file Taurus in the Constellations folder to see Taurus and to see Aldebaran, the Hyades, and the Pleiades. The Pleiades star cluster Section 5 - Deep Space 194 Spring Gemini (the Twins) are two bright stars - Castor and Pollux - and two strings of fainter stars that extend from them and that can be made to resemble two brothers standing sideby- side. Their exploits were legendary in the classical world. Castor is a sextuple star (two, which are bluish, are visible through a telescope) that lies 52 light years from earth. Yellowish Pollux lies 18 light years nearer to earth than Castor and the two are physically unrelated. Near Castors foot is the bright 5th-magnitude open star cluster M35. It is as large as the full moon and is easily visible to the unaided eye on a dark night, and it is a pretty sight in binoculars. The cluster, which contains at least 200 stars, lies 2,700 light years from earth and is 20 light years in diameter. Open the file Gemini in the Constellations folder to see Gemini and to see Castor, Pollux, and M35. Cancer (the Crab) is a small constellation with faint stars only, and it cannot be seen from within a city. Most people have heard of it only because the sun, moon, and planets pass through it. Cancers major feature of interest is the Beehive Star Cluster (also called the Praesepe - Latin for manger). At 3rd magnitude, it can easily be seen without binoculars, and with a diameter of more than 1 it is too large to fit within the field of view of most telescope eyepieces. The Beehive contains at least 50 stars, is 13 light years in diameter, and is 580 195 Spring Constellations light years from earth. The Beehive was one of four star clusters known in antiquity (the others being the Hyades, Pleiades, and Coma Berenices). Open Cancer to see Cancer and the Beehive Star Cluster. Zoom in to see the Beehives brightest stars. Coma Berenices (Berenices Hair) was named in honor of Queen Berenice of ancient Egypt. It lies one-third of the way between the tail of Leo and the end of the handle of the Big Dipper Coma Berenices is both a constellation and a star cluster. To the unaided eye it is an indistinct cluster of faint stars almost five degrees across - almost as large as the bowl of the Big Dipper. Binoculars or a telescope reveals several dozen stars 290 light years distant. Open the file Coma Berenices to see the constellation; zoom in to see the individual stars which make up the open cluster (it is in the bottom right corner, just beneath Gamma Comae Berenices, which is the last star in the stick figure). Note how the distances of these stars are all around 290 or 300 light years - if a stars distance is much different from this, it is not part of the cluster. Ursa Major (the Great Bear) is a huge constellation (the third largest) that lies far to the north and is best seen in the northern hemispheres spring and summer. The most famous part of it are the seven brightest stars which form the Big Dipper (as it is called in the United States and Canada), but the rest of the bear extends to the west and south. M44, the Beehive Cluster. Section 5 - Deep Space 196 Big Dipper The Big Dipper is so obvious and well known that you might think it has always been called that - but not so. It is called a dipper only in the United States and Canada, and there it has been called a dipper for only about 200 years. Its origin is lost in time, but the idea of a dipper might have come with slaves from Africa, where they drank from hollowed gourds. Before water came from taps, people used dippers to scoop water out of buckets. In England the Big Dipper is called the Plough, and in Germany it is the Wagon. Mizar is the second star from the end of the handle of the Big Dipper, and is actually part of a multiple star system, as we saw in chapter 5.1. People with better-than-average eyesight will see a slightly fainter star, Alcor, 12 arcminutes distant. The two names come from horse and rider in old Arabic, and according to legend they were used as an eye test. Mizar is 78 light years distant and Alcor lies 3 light years beyond. Mizar itself is a very pretty double star in a small telescope. It has a 4th-magnitude companion star that lies 14 arcseconds distant and that is an easy target for even the smallest telescopes. Their true separation is at least 10 times the distance from the sun to Pluto. Mizar was the first double star discovered, in 1650. Open the file Ursa Major in the Constellations folder to see the Big Dipper and Mizar and Alcor. 197 Summer Constellations Summer Cygnus (the Swan) is a graceful bird with outstretched wings in classical mythology. His head is the star Albireo, his tail the bright white star Deneb, and his wings reach symmetrically to the side. Cygnus was renamed the Cross by Schiller in his unsuccessful attempt to Christianize the constellations, and the name stuck - although it is unofficial. It is often called the Northern Cross (equally unofficially) to distinguish it from the Southern Cross. Cygnus straddles the Milky Way in the summertime. Use binoculars or a low-power telescope to see the myriad of faint stars within it. The Milky Way is especially rich in this direction because when we look towards Cygnus, we are looking down the length of one of the spiral arms. The dark Great Rift - a split in the Milky Way that begins north of the middle of Cygnus and that continues far to the south - is the effect of enormous dark clouds of gas and dust that block the light of millions of faint stars that lie beyond Two especially noteworthy stars are Deneb and Albireo. Deneb, the brightest star in the constellation and one of the stars of the Summer Triangle, is a true supergiant. Its blue-white color distinguishes it from other giants which are orange, such as Antares and Betelgeuse. Deneb is 50,000 times as luminous as our sun. It lies at the enormous distance of 1,600 light years - so far away that the light we see tonight left Deneb at the end of the Roman Empire. Albireo, which marks the Swans head, is a pretty double star in a small telescope with contrasting colors (often described as blue and gold) separated by a comfortable 34 arcseconds. They are giant stars 400 light years from earth and at least 650 billion kilometers (100 times the distance from Pluto to the sun) from each other. Open the file Cygnus to see the constellation outline and stick figure with bright stars highlighted, and the Milky Way. Section 5 - Deep Space 198 Hercules (the Strong Man) is the ancestor of Superman and other superheroes who have extraordinary strength and skill. He was formerly called the Kneeler for reasons long lost. Hercules is a rough box of mediumbright stars - the Keystone - plus others nearby that can be made to look like a man only with great difficulty. Hercules contains the famous M13 Globular Star Cluster, one of the showpieces of summer star parties since it passes nearly overhead and is easy to find. Look for it 1/3 of the way from Eta to Zeta Herculis on the western segment of the Keystone. At 6th magnitude it is barely visible to the naked eye under ideal conditions, and through a small telescope it looks like a small fuzzy patch of light. A large telescope reveals that it is made of thousands of stars, the brightest of which are 12th magnitude. The cluster is about 23,000 light years distant and about 150 light years in diameter. Open the file Hercules to see Hercules and the globular cluster M13. Lyra (the Lyre) is a small constellation whose brightest star, Vega, is the westernmost of the three stars of the Southern Triangle. A lyre is an ancient stringed instrument which is the ancestor of the harp and the guitar. Lyra is overhead during the summer in North America. Vega is the fifth brightest star in the sky. It is also relatively nearby at a distance of 25 light years (55 trillion kilometers or 35 trillion miles). It is a large white star M13, the Hercules Cluster 199 Summer Constellations about 50 times as luminous and 2-1/2 times as massive as our sun. Precession of the equinoxes will cause it to become the Pole Star in 14,000 AD. Vega is consuming its fuel rapidly and will burn out in less than one billion years. Near Vega is the famous double-double star Epsilon Lyrae. People with near-perfect eyesight can see that this 5th-magnitude star is actually two stars separated by a wide 209 arcseconds. Under magnification, each star is revealed to be a pair stars separated by a close 2-1/2 arcseconds. Each star is approximately 100 times the luminosity of our sun, and they are about 130 light years from earth. The famous Ring Nebula (M57) is the best known planetary nebula (see Chapter 5.1) - a tenuous shell of hot gas expelled from a dying star. Through a telescope it looks like a tiny slightly-oval donut just over 1 arcminute in diameter. Its true diameter is 1/2 light years and its distance is over 1,000 light years. Open the file Lyra to see Lyra, the stars Vega and Epsilon Lyrae, and the Ring Nebula. Sagittarius (the Archer) is a centaur, according to mythology that dates to Sumerian times. Centaurs, who were halfman and half-horse, combined the skill of men with the speed of horses and were not to be messed with. Today Sagittarius is easily seen as the outline of a teapot. It lies in the southern sky during summer. The constellation lies in front of an exceptionally rich part of the Milky Way, whose center lies within its boundaries, and it contains a wealth of objects for The Ring Nebula, M57 Section 5 - Deep Space 200 amateurs with telescopes. Only four are described here. The Lagoon Nebula (M8) is a 5th-magnitude glowing cloud of hot gas larger than the full moon and bright enough to see with the naked eye on a dark summer night. Only one other nebula (the Orion Nebula, which is visible in the opposite season) is more spectacular through a telescope. It fluoresces due to the presence of hot young stars near it that recently formed out of its gaseous material; a cluster of new stars nearby is called NGC 6530. The nebula is about 5,000 light years distant and about 50 by 100 light years in dimension. The Omega Nebula (M17) is so named because its arc-like shape resembles the capital Greek letter Omega. Also called the Swan or Horseshoe, this fluorescing diffuse nebula is bright enough to be visible in binoculars. It is about 6,000 light years distant and 40 light years across. The Trifid Nebula (M20) is smaller than the nearby Lagoon, but it is easier to see in a telescope because it has a higher surface brightness. Its name comes from a complex dust lane that lies in front of and trisects it. The Trifid is just 1-1/2 from M8 and the two can be seen together in a spotting scope or binoculars. The Trifid lies 6,500 light years from earth and is about 30 light years in diameter. M22 was the first globular cluster discovered, in 1665. It is visible to the naked eye on a dark night, and through a telescope it rivals M13 in Hercules. M22 would be more magnificent than M13 for observers in the United States and Canada were it higher in the sky, but its southern declination puts it at a disadvantage. M22 is one of the closest globulars with a distance of 10,500 light years from earth, and its brightest stars are 11th magnitude - a full magnitude brighter than the stars of M13. The Omega Nebula 201 Summer Constellations M22 is one of several bright globular clusters in Sagittarius (others are M28, M54, M55, M69, M70, and M75). Early this century the astronomer Harlow Shapley proposed that globular clusters are concentrated towards Sagittarius because they orbit the center of the Milky Way, and they all pass through this part of the sky before dispersing to the outer parts of their orbits. He concluded that the Milky Ways center must lie in the direction of Sagittarius and estimated the distance to it, displacing the earth from its apparent position at the center of our galaxy. Open the file Sagittarius to see the constellation and to see the globular clusters and nebulae within it. The Closest Galaxy The closest galaxy to the Milky Way is the Sagittarius Dwarf Elliptical Galaxy (SagDEG) which is half as distant as the two Magellanic Clouds. It circles the Milky Way with an orbital period of about one billion years. The 8th-magnitude globular star cluster M54, long believed to lie within the Milky Way, was found in 1994 to be at the enormous distance of 80,000 light years. This puts it outside the limits of our Milky Way but inside the SagDEG. It is the most distant globular star cluster visible in all but the very largest amateur telescopes and intrinsically the most luminous. Its brightest stars are 14th magnitude. M54, a star cluster in the nearest galaxy to our own. Section 5 - Deep Space 202 Scorpius (the Scorpion) has been called a scorpion for at least the last 6,000 years, and its stars do indeed look like a scorpion. Too far to the south to see well from Canada, it is prominent in the south in the summer sky from the southern United States. At one time its claws included the stars of Libra, to the west. The Scorpions heart is a bright red star (actually orange) called Antares - the rival of Mars in Greek because it rivals Mars in color and is often close to Mars in the sky (see the sidebar below). Antares is a true supergiant star 10,000 times as luminous as our sun and so large that if it were placed where our sun is, Mars would orbit beneath its surface. It lies about 500 light years from earth - the same distance as the orange star Betelgeuse in Orion. Like all giant stars, it varies slightly in brightness in an irregular pattern and will soon - astronomically speaking - burn out. Beta Scorpii (also known as Graffias) is an exceptionally pretty double star. The two stars are magnitudes 2.6 and 4.9 with a separation of 14 arcseconds, and it closely resembles Mizar in the Mars and Anti-Mars Together Antares lies near the ecliptic at an ecliptic latitude of -4 34, and the sun, moon, and planets pass near it. Mars is in conjunction with its rival Antares on the average of once every two years. Mars-Antares conjunctions occur in March 2001, January 2003, January 2005, December 2006, November 2008, November 2010, and so on. Open the file Antares and step through time to see the August 23, 2016 conjunction. If you wish, you can continue to run time forward or backward to see other conjunctions. 203 Summer Constellations Big Dipper. The pair is 600 light years distant. The brightest globular star cluster in Scorpius is M4. It is magnitude 5-1/2 and it lies only 1-1/3 west of Antares, so it is very easy to find. This exceptionally close (for a globular!) star cluster is 7,000 light years distant and 55 light years in diameter. Its brightest stars, like M22s in Sagittarius, are 11th magnitude. Two especially bright open star clusters are M6 and M7, which lie above the stinger of the Scorpions tail. At magnitudes 5 and 4 respectively, they are easily visible to the unaided eye if Scorpius is high enough in the sky from your latitude, and both can be seen together in binoculars. They are 2,400 and 800 light years distant and each contains about 80 stars. Ptolemy called M7 a nebula, and its identity as a star cluster was not known until the invention of the telescope. Because these clusters have very large angular sizes, observe them with very low magnification. Open the file Scorpius to see Scorpius and to see Antares, Beta Scorpii, and the star clusters M4, M6, and M7. Autumn Andromeda (the Princess) is a line of three bright stars and a string of fainter stars that forms a long narrow V in the autumn sky. Alpha Andromedae is one of the four stars that forms the Square of Pegasus. Andromeda contains one of the prettiest double stars in the sky - Gamma Andromedae, also called Almach. The two stars, which are magnitudes 2.2 and 4.9, are noted for their contrasting colors - yellow and bluish respectively. This color contrast is shown well with Starry Night. These are giants 90 and Section 5 - Deep Space 204 15 times the luminosity of our sun. The angular separation between them is 10 arcseconds (1/6 of 1/60 of one degree) and their distance from earth is 350 light years. To cosmologists, the constellation Andromeda is almost synonymous with the Andromeda Galaxy - the closest and brightest large galaxy to us. It is visible to the unaided eye on a dark night as a small fuzzy patch of light, but even with a large telescope it remains a disappointingly shapeless blob. We see only the inner part of what time-exposure photographs reveal to be a huge spiral galaxy of hundred of billions of stars that spans several degrees of the sky. It is similar in size and shape to the Milky Way; our Milky Way would look something like the Andromeda Galaxy from a similar distance. At a distance of almost 3 million light years, it is the most distant object you can see without a telescope. It has two satellite elliptical galaxies, M32 and NGC 205, which are easily visible in small telescopes. The Andromeda Galaxy is also known as M31 - the 31st object in Charles Messiers catalog of comet-like objects. Open the file Andromeda to see the constellation outline and stick figure with its major stars and M31 highlighted. Cepheus (the King) is an inconspicuous five-sided figure next to Cassiopeia in the far northern sky. From the northern United States it is circumpolar, and it never sets. Although just a faint star to the unaided eye, Delta Cephei is one of the most important and most studied stars of all. It is the prototype of the famous M31, the Andromeda Galaxy 205 Autumn Constellations Cepheid Variable Stars - unstable giant stars that pulsate in size and that vary in brightness in a regular way. The time it takes a Cepheid star to go from its maximum brightness down to its minimum brightness and back to its maximum again is called its period. The intrinsic brightness of these stars is related to the lengths of their periods, and this makes them a powerful tool for finding distances to galaxies. Because there is a one-to-one relationship between the period of variability and the brightness of a Cepheid variable, once its period is measured its intrinsic brightness is known, and since we can observe how bright the Cepheid appears to be, we can calculate its actual distance. The largest telescopes can see Cepheid Variables in galaxies 50 million light years distant. Delta Cephei is approximately 3,000 times more luminous than our sun and it lies some 1000 light years from earth. It varies in brightness by a factor of two from magnitude 3.6 to 4.3 in a period of precisely 5 days, 8 hours, and 48 minutes. Compare it to nearby 4.2-magnitude Epsilon Cephei and 3.4-magnitude Zeta Cephei to monitor its regular change in brightness from your backyard. Open the file Cepheus to see Cepheus in stick figure outline, and Delta Cephei. Perseus (the Hero) is a scraggly stream of stars near Cassiopeia and Andromeda in the autumn sky. Algol is the demon star, its name coming from Arabic for the ghoul. By the time of the ancient Greeks, observers were alarmed that the star brightens and fades in a regular pattern. Today we know that the star is actually two stars in orbit around a common center of gravity and so aligned that they eclipse each other as seen from earth. Normally they appear as a single star of magnitude 2.1, but when the brighter star is hidden by the fainter, it fades to magnitude 3.4 for about 10 hours. The entire Section 5 - Deep Space 206 cycle lasts 2 days, 20 hours, and 49 minutes. Compare with the nearby star Gamma Andromedae, which remains at a constant magnitude 2.1, and Epsilon Persei, which is magnitude 2.9. Algol is about 100 light years distant. (see the section on variable Stars in Chapter 5.1) The Perseus Double Star Cluster is one of the more striking naked-eye deep-space objects in the sky. Visible to the eye as two out-of-focus stars that lie near to each other in the Milky Way, binoculars reveal them to be two giant but distant clusters of hundreds of stars. Their centers are 1/2 apart, but their edges overlap. The clusters are each magnitude 4-1/2, and their brightest stars are 6th magnitude. At a distance of about 7,200 light years, these stars are true giants with 100,000 times the luminosity of our sun. The clusters lie in the Perseus Arm of the Milky Way - the next spiral arm beyond the Orion Arm. Open the file Perseus to see Perseus and Algol. The Perseus double cluster is not included in Starry Night Backyard as a separate object, but this file is set up so that your view is centred on the cluster. If you zoom in to a field of view of about 4, you will see two bright clusters of stars just below the centre of the screen. If you have Starry Night Pro, you can turn on the outlines of the two clusters by choosing Sky | NGC/IC. The Southern Sky As we learned in chapter 3.2, the Equator is the only place on earth where all stars are above the horizon at some time. The farther north or south that we move from the equator, the more stars are forever hidden to us. The constellations described in this section are located south of the celestial equator and cannot be seen well (if at all) in the North. 207 Southern Sky Constellations Carina (the Keel) is part of the classical Ptolemaic constellation Argo, the ship. Lacaille subdivided this enormous ancient constellation into three, of which Carina is the southernmost part. Canopus, the second brightest star in the sky (after Sirius), is named after the pilot of the fleet of Spartan ships that sailed to Troy to recover Queen Helen. It is barely visible from the southernmost parts of the United States. Canopus is a yellowish supergiant star with a diameter 65 times that of our sun, a luminosity of 15,000 suns, and a distance of 310 light years from earth. The giant and explosively variable star Eta Carinae is surrounded by a nebula where stars are now forming. The Eta Carinae Nebula - the largest in the sky - is visible to the naked eye as a huge splotch of light 2 across. It is over 9,000 light years from earth. A dark cloud in front of it is called the Keyhole Nebula. Open the file Carina to see Canopus and the star Eta Carinae. Although an image of the Eta Carinae nebula is not included in Starry Night, you can select the star Eta Carinae and then choose Edit | Online Info (Selection | Online Info in Starry Night Pro) to get information and images of this nebula. Centaurus (the Centaur) lies well below the celestial equator. Its northern part can be seen from the southern States, but its southernmost stars cannot be seen from the United States (other than Hawaii). The closest star to earth, other than the sun, is the triple star system collectively known as Alpha Centauri. It lies 4.3 light years from earth, or 300,000 times as far as our sun. Alpha Centauri A (also known as Rigil Kentaurus) has a bright companion (Alpha Centauri B) which orbits in an 80- Section 5 - Deep Space 208 year period; they presently are 14 arcseconds apart and a magnificent pair in a small telescope. Another companion, Proxima Centauri, is a faint red dwarf star trillions of light years distant from the main two and slightly closer to earth than the main pair. Beta Centauri (also known as Hadar) lies less than 5 to the west of Alpha, and the two are a striking pair of very bright stars near the Southern Cross. The 4th-magnitude Omega Centauri Globular Star Cluster is the biggest and most luminous globular star cluster in the Milky Way (it has perhaps a million stars) and also the brightest in our sky. Its nature was discovered in 1677 by Edmund Halley (of Halleys Comet fame); earlier it was mistakenly considered a star, which is why it has a Bayer letter. To the naked eye it is a fuzzy ball of light, but through a small telescope it is a stunning sight. It lies approximately 50,000 light years from earth. There is not an image of Omega Centauri in Starry Night, but Starry Night Pro owners can locate its position by choosing Selection | Find and typing in Omega Centauri. Open the file Centaurus to see Centaurus and Alpha and Beta Centauri. Crux (the Southern Cross) is the smallest constellation. The compact group of five bright stars is the best-known star pattern in the southern sky. It guides you to where the South Star would be if there were one: follow the long arm of the cross from Gamma through Alpha and continue it 4-1/2 cross-lengths (27) southward to find the South Celestial Pole. The Southern Cross is a familiar icon in Australia and New Zealand, where you will see it on everything from flags and stamps to beer. It is visible from Hawaii and points south. The Jewel Box Open Star Cluster (also knows as the Kappa Crucis Cluster) is a large 4th-magnitude open star cluster that is 209 Southern Sky Constellations easily visible to the naked eye and very pretty in binoculars. The Jewel Box is 20 light years across and about 7000 light years distant, and it contains at least 200 stars, the brightest of which are 7th magnitude. One of the most unusual objects in Crux is what looks like a hole in the Milky Way that is about 5 in diameter. The Coal Sack Dark Nebula is a huge cloud of dark dust, 20 to 30 light years across and perhaps 600 light years distant, that blocks the light of stars beyond. It is the most conspicuous of the many dark clouds that lie along, and partially obscure, the Milky Way. Open the file Crux to see the Southern Cross, the Coal Sack, and the Jewel Box. To find the Jewel Box, zoom in on the star labeled Mimosa. Then zoom in to a field of view of about 5. The Jewel Box is visible as a patch of stars about 1.5 directly above Mimosa. The Coal Sack is the region with very few stars which is up and to the left of the Jewel Box. This concludes our brief tour through the constellations, but it barely scratches the surface. The night sky is filled with uncountable wonders, and with a telescope you could explore the constellations forever. Starry Night Companion 210 CONCLUSION The sky is a fascinating place - especially if you understand it. It is so removed from our normal experiences that celestial objects and their slow movements can seem too abstract to relate to. Some people are overwhelmed and conclude that understanding the sky is beyond them; others (like you) put in the time and trouble to puzzle it all out. You have a lot of help in the form of this book and software package. When I was a youngster, I taught myself the constellations through a few books and a rotating star finder, and I learned how objects in the sky move by reading about them and by studying diagrams in books. I had to do a lot of mental conversions to connect words, static illustrations, and tables of figures with what I saw in the sky, but in time I figured it out. It was not until desktop astronomy software came along many years later that I was really able to visualize, for the first time, what these movements looked like. I had thought about the retrograde loop of Mars and had followed it Starry Night Companion 212 through the months - but now I could see it in minutes. I had thought about why eclipses did not happen each month - but now I could see how the paths of the sun and moon intersect and how that intersection point moves from year to year. I had thought about how precession causes the vernal equinox to shift - but now I could skip through centuries and watch the equinox precess. I had thought about how the planets would appear to orbit as seen from the sun - but now I could put myself at the sun and watch them move. To one who learned it the hard way, this was magical. You, dear reader, are fortunate to have a tool such as Starry Night to help you understand the sky. Work with it, play with it, fool around with it, and gain wisdom. The reward is a feeling of being connected with the sky. Good luck, and may you never stop learning about the sky. May you be at home under it wherever you go. - John Mosley APPENDIX A THE CONSTELLATIONS Common Name Latin Name Abb. Possessive Princess Andromeda And Andromeda Air Pump Antlia Ant Antliae Bird of Paradise Apus Aps Apodis Water Carrier Aquarius Aqr Aquarii Eagle Aquila Aql Aquilae Altar Ara Ara Arae Ram Aries Ari Arietis Charioteer Auriga Aug Aurigae Herdsman Botes Boo Bootis Chisel Caelum Cae Caeli Giraffe Camelopardalis Cam Camelopardalis Appendix 214 Common Name Latin Name Abb. Possessive Crab Cancer Cnc Cancri Hunting Dogs Canes Venatici CVn Canum Venaticorum Large Dog Canis Major CMa Canis Majoris Small Dog Canis Minor CMi Canis Minoris Sea Goat Capricornus Cap Capricorni Keel Carina Car Carinae Queen Cassiopeia Cas Cassiopeiae Centaur Centaurus Cen Centauri King Cepheus Cep Cephei Sea Monster or Whale Cetus Cet Ceti Chameleon Chamaeleon Cha Chamaeleontis Compasses Circinus Cir Circini Dove Columba Col Columbae Berenices Hair Coma Berenices Com Comae Berenices Southern Crown Corona Australis CrA Coronae Australis Northern Crown Corona Borealis CrB Coronae Borealis Crow Corvus Crv Corvi Cup Crater Cra Crateris Southern Cross Crux Cru Crucis Swan Cygnus Cyg Cygni Dolphin Delphinus Del Delphini Swordfish Dorado Dor Doradus Dragon Draco Dra Draconis Colt Equuleus Equ Equulei 215 The Constellations Common Name Latin Name Abb. Possessive River Eridanus Eri Eridani Furnace Fornax For Fornacis Twins Gemini Gem Gemini Crane Grus Gru Gruis Strong Man Hercules Her Herculis Clock Horologium Hor Horologii Water Serpent (f) Hydra Hya Hydrae Water Serpent (m) Hydrus Hyi Hydri Indian Indus Ind Indi Lizard Lacerta Lac Lacertae Lion Leo Leo Leonis Small Lion Leo Minor LMi Leonis Minoris Hare Lepus Lep Leporis Scales Libra Lib Librae Wolf Lupus Lup Lupi Lynx Lynx Lyn Lyncis Lyre Lyra Lyr Lyrae Table Mountain Mensa Men Mensae Microscope Microscopium Mic Microscopii Unicorn Monoceros Mon Monocerotis Fly Musca Mus Muscae Carpenters Square Norma Nor Normae Octant Octans Oct Octantis Serpent Bearer Ophiuchus Oph Ophiuchi Hunter Orion Ori Orionis Peacock Pavo Pav Pavonis Appendix 216 Common Name Latin Name Abb. Possessive Flying Horse Pegasus Peg Pegasi Hero Perseus Per Persei Fire Bird Phoenix Phe Phoenicis Painters Easel Pictor Pic Pictoris Fishes Pisces Psc Piscium Southern Fish Piscis Austrinus PsA Piscis Austrini Stern Puppis Pup Puppis Mariners Compass Pyxis Pyx Pyxidis Reticle Reticulum Ret Reticuli Arrow Sagitta Sge Sagittae Archer Sagittarius Sgr Sagittarii Scorpion Scorpius Sco Scorpii Sculptors Apparatus Sculptor Scl Sculptoris Shield Scutum Sct Scuti Serpent Serpens Ser Serpentis Sextant Sextans Sex Sextantis Bull Taurus Tau Tauri Telescope Telescopium Tel Telescopii Triangle Triangulum Tri Trianguli Southern Triangle Triangulum Australe TrA Trianguli Australis Toucan Tucana Tuc Tucanae Great Bear Ursa Major UMa Ursae Majoris Small Bear Ursa Minor UMi Ursae Minoris Sails Vela Vel Velorum Virgin Virgo Vir Virginis Flying Fish Volans Vol Volantis Fox Vulpecula Vul Vulpeculae 217 The Constellations The Common Name is often the English equivalent of the Latin name, but it can be a descriptive term too (as in the Twins for Gemini). The Abb. is the official three-letter abbreviation. The possessive of form is used following a stars name, as in Alpha Centauri - the Alpha star of Centaurus. Those who studied Latin might recall that the possessive form is the Latin genitive case. Appendix 218 APPENDIX B PROPERTIES OF THE PLANETS Distance From Sun (AU*) (Earth radii) (Earth masses) of Year (Earth years) of Day (Earth solar days) of Orbit to the Ecliptic Plane () Planet Mean Radius Mass Length Length Inclination Mercury 0.39 0.38 0.06 0.24 175.94 7 Venus 0.72 0.95 0.82 0.62 117 3.4 Earth 1 1 1 1 1 0 Mars 1.52 0.61 0.11 1.88 1.03 1.9 Jupiter 5.20 11.21 317.83 11.86 0.41 1.3 Saturn 9.55 9.50 95.16 29.42 0.44 2.5 Uranus 19.22 8.01 14.50 83.75 0.72 0.8 Neptune 30.11 7.77 17.20 163.7 0.67 1.8 Pluto** 39.54 0.36 0.002 248.0 6.39 17.1 * 1 AU is the average earth-sun distance, which is 149 600 000 km (92 900 000 miles) ** Pluto is sometimes no longer considered a planet Glossary 220 GLOSSARY absolute magnitude: the apparent brightness (magnitude) a star would have if it were 32.6 light years (10 parsecs) from the earth. It is used to compare the true, intrinsic brightnesses of stars. The sun has an absolute magnitude of +4.8. annular eclipse: solar eclipse where the moon passes directly in front of the sun, but is too far from the earth to completely cover the sun. A ring of sunlight surrounds the moon at the peak of the eclipse. Antarctic Circle: the line of latitude on the earths surface that is 23-1/2 north of the South Pole. The Antarctic Circle marks the northernmost points in the Southern Hemishpere that experience the midnight sun. aphelion: the point in an objects orbit where it is farthest from the sun (helios = sun). The earth is at aphelion each year on about July 3. Glossary 222 arcminute: a unit of angular measure equal to one sixtieth of a degree. As an example, the moon has an apparent diameter of about 30 arcminutes. arcsecond: a unit of angular measure equal to one sixtieth of one sixtieth of a degree, or one sixtieth of an arcminute. As an example, the apparent diameter of Jupiter is about 45 arcseconds. Arctic Circle: the line of latitude on the earths surface that is 23-1/ 2 degrees south of the North Pole. The Arctic Circle marks the southernmost points in the Northern Hemisphere that experience the midnight sun. apparent magnitude: a system used to compare the apparent brightness of celestial objects. The lower an objects apparent magnitude, the brighter it is. A change in magnitude of 1 corresponds to a change in brightness by a factor of 2.5. Objects with a magnitude of less than 6 can be seen with the naked eye in good observing conditions. asterism: a group of stars that people informally associate with each other to make a simple pattern, such as the Big Dipper and Square of Pegasus. The stars in an asterism can come from one or more official constellations. asteroid: one of the many thousands of chunks or rock or iron that orbit the sun, also known as minor planets (an older term). Most orbit between Mars and Jupiter, where they formed, but some cross the orbit of the earth. Fragments of asteroids are called meteorites if they fall to the ground. astrological sign: one of 12 sections of the zodiac that are 30 long and that corresponds to the positions of the constellations as they were about 2600 years ago when the astrological system was established. Do not confuse an astrological sign with an astronomical 223 Glossary constellation with the same or a similar name, as they only partially overlap. autumnal equinox: the moment when the sun crosses the ecliptic in a southward direction on or about September 22. In the Northern Hemisphere, it marks the first day of autumn. It is also the suns position in the sky at that moment. In the sky, it is one of two intersection points of the ecliptic and celestial equator (the other being the vernal equinox). binary star: two stars which orbit a common center of gravity. These stars often look like a single star to the naked eye. If more than two stars orbit a common center of gravity, it is called a multiple star. birth sign: see astrological sign. celestial equator: the projection of the earths equator into space; also a line in the sky midway between the North and South Celestial Poles. The celestial equator is the line of zero declination in the equatorial co-ordinate system. celestial meridian: The line of zero right ascension in the equatorial co-ordinate system. celestial sphere: The projection of the earth into space. The stars can be imagined to be drawn on the inside of this sphere. circumpolar: those stars that are so far north (or so far south, in the Southern Hemisphere) they do not set at all as seen from a given latitude. comet: a body composed of ice and dust in orbit around the sun. conjunction: the passing of one planet by another planet or by the moon or sun. Two planets are in conjunction when they have the same ecliptic longitude (or alternately the same right ascension). Glossary 224 constellation: one of the 88 portions of the sky that are officially recognized by the International Astronomical Union (see Appendix A). A constellation is an arbitrary area of the sky, and it includes everything within that areas boundaries, regardless of distance from the earth. Daylight Saving Time: the adjustment to the clock time that is put into effect during the summer to extend daylight one hour later in the evening. In the United States and Canada, Daylight Saving Time begins on the first Sunday in April (set clocks ahead one hour) and ends on the last Sunday in October (set clocks back one hour); other countries change on different dates. Daylight time is one hour ahead of the equivalent standard time ( 6 p.m. standard time becomes 7 p.m. when daylight time is in effect). declination: the angular distance of an object north of (positive) or south of (negative) the celestial equator, expressed in degrees. It is the celestial equivalent of latitude on the earths surface. The declination of the celestial equator is 0; the declination of the North Celestial Pole is +90, and the declination of the South Celestial Pole is -90. double star: two stars that appear near each other in the sky. Their apparent closeness may be due to chance alignment, with one star far beyond the other, or they may be in orbit around a common center of gravity, in which case they form a binary star. eclipse: the passage of one object in front of another (as the moon passes in front of the sun during an eclipse of the sun), or the passage of one object through the shadow of another (as the moon passes through the shadow of the earth during an eclipse of the moon). eclipse season: the 38-day period when the sun is near a node of the moons orbit and one or more solar eclipses may happen. 225 Glossary ecliptic: the suns apparent annual path through the fixed stars; also the orbit of the earth if it could be seen in the sky. The 13 astronomical constellations that the ecliptic passes through are the astronomical constellations of the zodiac (12 in astrology). ecliptic co-ordinate system: the system of specifying positions in the sky that uses the ecliptic - the suns apparent path around the sky - as the fundamental reference plane. Ecliptic co-ordinates are useful when specifying positions in the solar system and especially positions relative to the sun. ecliptic latitude: the angular distance of an object above (positive) or below (negative) the ecliptic, expressed in degrees. The ecliptic latitude of the sun is always zero. ecliptic longitude: the angular distance of an object, measured along the ecliptic, eastward from the vernal equinox, and expressed in degrees. The ecliptic longitude of the sun is 0 when the sun is on the vernal equinox, and it increases by very nearly 1 per day through the year. elliptical galaxy: one of the two major types of galaxies, the other being a spiral galaxy. They often resemble star clusters when seen from the earth. epoch: particular date for which astronomical positions in a book or table are accurate. Most books give star positions which are valid for the J2000 (January 1, 2000) epoch. equation of time: the difference between true solar time (determined by the suns position in the sky) and mean solar time (the time told by your watch). The two times can vary by as much as 16 minutes over the course of a year. equatorial co-ordinate system: a system of specifying positions in the sky that uses the celestial equator - the projection of the earths equator into space - as the fundamental reference plane. Glossary 226 field of view: angular width of sky that can be seen with an optical instrument. Field of view is measured in degrees, arcminutes, and arcseconds. Foucault pendulum: pendulum which varies the direction of its swing as the earth rotates. Used to demonstrate that the earth rotates, not the sky. full moon: the moon when it lies directly opposite the sun. The moon is full two weeks after new moon. The full moon rises at sunset and sets at sunrise. The earth is between the full moon and the sun. galactic co-ordinate system: the system of specifying positions in the sky that uses the plane of the Milky Way as the fundamental reference plane. globular cluster: a huge spherical cluster of tens of thousands of stars. The stars of a cluster were born together and travel through space together. M13 and M22 are familiar examples. greatest eastern elongation: the greatest angular distance to the east of the sun reached by Mercury or Venus. When a planet is at its eastern elongation, it sets after the sun and is at its best visibility in the evening sky. inferior conjunction: the passage of Mercury or Venus between the earth and the sun. The outer planets cannot pass between the earth and the sun and cannot come to inferior conjunction. inferior planet: Mercury or Venus, so-called because their orbits are inside the earths orbit around the sun and inferior to the earth in terms of distance from the sun. Julian Day: the number of days (and fractions of days) that have elapsed since noon, Jan.1, 4713 BC (Greenwich Mean Time). It is used to simplify calculating the time interval between two events. 227 Glossary For example, 9:00 p.m. P.S.T. on January 1, 2000, was Julian day 2,451,545.71. latitude: the angular distance of an object north or south of the earths equator expressed in degrees. The latitude of the equator is 0, of Chicago is 42 North, the North Pole is 90 North, and Lima, Peru is 12 South. Latitudes south of the equator are expressed as South or negative. light year: the distance light travels in one year. One light year equals 9,460,536,000,000 kilometers or 5,878,507,000,000 miles. limiting magnitude: the magnitude of the dimmest object that can be seen through an optical instrument (including the eye). The limiting magnitude of an instrument will vary with light conditions. light pollution: the brightening of the night sky due to artificial light. Light pollution makes it impossible to view many dim objects that can only be seen in a dark sky. locked rotation: condition where a moon has the same period of rotation as its period of revolution around its parent body. This means that the moon always shows the same face to its parent planet. Our moon is in locked rotation around the earth. longitude: the angular distance of an object east or west of the Prime Meridian (the line of zero longitude which runs through Greenwich England), expressed in degrees. The longitude of Chicago is 88 West. luminosity: an expression of the true brightness of a star as compared to the sun. The suns luminosity is 1.0 by definition. Sirius has a luminosity of 23 and Rigel a luminosity of about 50,000. lunar eclipse: an eclipse of the moon (lunar = moon), caused when the moon moves partially or wholly into the shadow of the earth and grows dark for up to a few hours. A lunar eclipse can be seen by everyone on the side of the earth facing the moon. Glossary 228 magnitude: an expression of the brightness of a star (or other celestial object) as it appears from the earth, according to a system devised by Hipparchus. Also known as apparent magnitude, to distinguish from absolute magnitude.Larger numbers refer to fainter stars, and the brightest stars and planets have negative magnitudes. One magnitude difference is equal to a brightness difference of 2.5 times. massing: close alignment of three or more planets (or two or more planets and the moon), as seen from earth. This occurs when all the bodies involved in the massing have similar ecliptic longitudes. meridian: the line in the sky that extends from the southern point on the horizon through the zenith (overhead point) to the northern point on the horizon, bisecting the sky into an eastern and western half. Objects are at their highest when they cross the meridian; the sun is on the meridian at local noon. Also a line on the surface of the earth (or another body) that extends from pole to pole. Messier object: one of the 110 objects in the catalog compiled by Charles Messier. Most Messier objects are galaxies, star clusters or nebulae. meteor: the visible flash of light produced when a meteorite falls through the atmosphere and bursts into flame because of friction with air molecules; also called a shooting star or falling star. meteorite: the solid particle, either stone or iron, that falls through the atmosphere to produce a meteor. Most meteorites are fragments of asteroids. Science museums display meteorites that survived their falls. midnight sun: the sun when visible at midnight, which happens only in summer north of the Arctic Circle or south of the Antarctic Circle. 229 Glossary Milky Way: our own galaxy. minor planet: see asteroid. month: the period of time it takes the moon to orbit the earth (or a moon to orbit a planet). A sidereal month (27-1/3 days) is the time it takes the moon to orbit the earth and return to the same position relative to the stars; a synodic month (29-1/2 days) is the time it takes the moon to orbit the earth and return to the same position relative to the sun and is the time between new moon and new moon. nebula: a cloud of gas or dust in space, either between the stars or expelled by a star; nebula is Latin for cloud. There are many kinds of nebulas. new moon: the moon when it lies in the same direction as the sun and the beginning of a cycle of lunar phases. The new moon rises and sets with the sun. The moon is between the earth and the sun at new moon. night vision: enhanced ability to see objects in the dark. Night vision is ruined if the eyes are exposed to bright light. node: the point(s) in the sky where two orbits or paths cross. The nodes of the moons orbit are the two places where the moons orbit crosses the ecliptic. North Celestial Pole: the skys north pole; the point in the sky directly above the earths north pole. obliquity of the ecliptic: the amount of the tilt of the earths axis (23.5) which determines the angle the ecliptic makes with the celestial equator as they intersect in the sky. occultation: the disappearance (eclipse) of one object behind another, as a star or planet behind the moon or a star behind a planet. Glossary 230 open cluster: a diffuse association of a few dozen to a few thousand stars, all of which were born together and which travel through space together. The Pleiades in Taurus and Beehive in Cancer are familiar examples. opposition: the position of a planet when it is opposite the sun in the sky. Only objects that orbit outside the earths orbit come into opposition; Mercury and Venus cannot. parallax: the apparent shift in position of an object when it is viewed from two different points. The parallax of a star is measured from opposite ends of the earths orbit, and for the nearest stars is less than one second of arc (one arcsecond). parsec: the distance an object would have if its parallax were one second of arc (see parallax). One parsec equals 3.26163 light years or 30,856,780,000,000 kilometers. penumbra: shadowed area in an eclipse where only part of the light source is blocked. Observers in the penumbral shadow of a solar eclipse see a partial eclipse. perihelion: the point in an objects orbit when it is closest to the sun (helios = sun). The earth is at perihelion each year on about January 3. Period: the time an object takes to complete a certain motion and return to its original state, e.g. period of revolution planetary nebula: a luminous cloud of gas expelled by an aging star that has become unstable. The name comes from a nebulas superficial resemblance to the faint planets Uranus and Neptune as seen through a small telescope. The Ring Nebula M57 in Lyra is a familiar example. precession of the equinoxes: the slow wobbling of the earths axis in a 25,800-year cycle, caused by the gravitational attraction of the 231 Glossary moon on the earths equatorial bulge. Precession causes the vernal equinox (and all other points on the ecliptic) to regress westward along the ecliptic, slowly changing the equatorial co-ordinate grid. Prime Meridian: the line of longitude which passes through Greenwich, England, and which is the zero line for expressing longitude on the earths surface. proper motion: the motion of the stars relative to each other, caused by their actual motion in different directions at different speeds through space. retrograde motion: the apparent backwards or westward motion of a superior planet against the background of the stars caused when the faster-moving earth on an inside orbit passes that planet. revolution: the orbiting of one body around another body, as the moon revolves around the earth and the earth revolves around the sun. right ascension: in the equatorial co-ordinate system, the angular distance of an object eastward from the zero point (which is the vernal equinox), usually expressed in hours and minutes (which represents the earths rotation from the vernal equinox to the object). It is the celestial equivalent of longitude on the earths surface. rotation: the spinning of a body on its own axis. The earth rotates once a day. See revolution, which is often confused with rotation. sidereal day: the time it takes the earth to rotate once relative to the stars, in 23 hours 56 minutes and 4 seconds, which is 4 minutes less than a solar day. (During one sidereal day the sun moves 1 east along the ecliptic, and the earth has to rotate 4 additional minutes to complete one rotation relative to the sun in one 24-hour solar day.) sidereal month: the time it takes the moon to complete one orbit of the earth and return to the same position among the stars, on an average of 27.32166 days. Glossary 232 sidereal year: the time it takes the earth to complete one orbit of the sun relative to the stars, in 365 days 6 hours 9 minutes and 10 seconds. It is also the time it takes the sun to appear to travel once around the sky relative to the stars. solar day: the time it takes the earth to spin once relative to the sun, in exactly 24 hours (by definition). solar eclipse: an eclipse of the sun by the moon, when the moon passes in front of the sun. Solar eclipses can be partial, total, or annular. Only the few people in the narrow path of totality see a solar eclipse as total. South Celestial Pole: the point in the sky directly above the earths south pole. spiral galaxy: The second major type of galaxy, characterized by a central bulge and a number of spiral arms extending from the bulge. standard time zone: one of the 24 sections of the earth, each about 15 degrees wide and extending from pole to pole, within which the time is the same. In practice, natural and political boundaries determine the edges of time zones. summer solstice: the moment when the sun reaches its greatest distance north of the celestial equator, on or about June 21. In the Northern Hemisphere this marks the first day of summer; in the Southern Hemisphere it marks the first day of winter. superior conjunction: the position of a planet when it is on the far side of the sun (and in conjunction with the sun). superior planet: the planets Mars through Pluto, so-called because their orbits are outside the earths orbit around the sun and thus superior to the earth in terms of distance from the sun. synodic month: the time it takes the moon to complete one cycle 233 Glossary of phases, such as from new moon to new moon, on an average of 29.53059 days. transit: the passage of one celestial body across the face of a second larger body. triple conjunction: close alignment of a planet and a star at three distinct times, caused by the retrograde motion of the planet. The planet passes the star once in its forward motion, once more in its retrograde motion, and a third time when it resumes its forward motion. tropical year: the length of time it takes the sun to circle the sky relative to the vernal equinox. The tropical year is identical to our standard year. Tropic of Cancer: the line of latitude on the earths surface that is 23-1/2 degrees north of the equator. It marks the northernmost points in the Northern Hemisphere from which the sun can appear directly overhead. Tropic of Capricorn: the line of latitude on the earths surface that is 23-1/2 degrees south of the equator. It marks the southernmost points in the Southern Hemisphere from which the sun can appear directly overhead. umbra: shadowed area in an eclipse where the light source is completely blocked. Observers in the umbral shadow of a solar eclipse experience a total eclipse. Universal Time: in simplest terms, the time at the longitude of Greenwich, England. Universal Time (UT) is widely used in international publications as a standard time. variable star: a star whose light changes over time. Stars can vary in brightness for a variety of reasons, from eclipses by companions to instability of their interiors that causes the stars to swell and shrink. Glossary 234 vernal equinox: the moment when the sun crosses the ecliptic in a northward direction on or about March 21. In the Northern Hemisphere, it marks the first day of spring. It is also the suns position in the sky at that moment. In the sky, it is one of two intersection points of the ecliptic and celestial equator (the other is the autumnal equinox). waning crescent: the phase of the moon between third quarter and new moon. Waning means declining or fading. waning gibbous: the phase of the moon between full moon and last quarter. waxing crescent: the phase of the moon between new moon and first quarter. Waxing means increasing. waxing gibbous: the phase of the moon between first quarter and full. winter solstice: the moment when the sun reaches its greatest distance south of the celestial equator, on or about December 22. In the Northern Hemisphere this marks the first day of winter; in the Southern Hemisphere it marks the first day of summer. zenith: the point in the sky directly above the observer; the top of the sky. zodiac: the band of constellations or signs that the sun passes through as it moves around the sky. There are 12 signs of the astrological zodiac but 13 constellations of the astronomical zodiac. INDEX A Absolute magnitude 158 Age of Aquarius 110 Albireo 197 Alcor 196 Aldebaran 192 Algol 205 Almach 203 Alpha Centauri 207 Altitude 20 best for observing 20 Andromeda 203 Andromeda Galaxy 171, 204 Angles between objects in the sky 30 measuring 22 Antarctic Circle 94 Antares 202 Apollo 11 mission 122 Arctic Circle 94 Aries 19 Asterisms 175 Asteroids 149 Astrology 107 Autumnal equinox 89 Azimuth 21 B Bayer 180 Beehive Cluster 194, 195 Beta Scorpii 202 Betelgeuse 190 Big Dipper 19, 188, 196 Binary stars 161 Binoculars, tips on purchasing 58 Birth signs 109 Black hole 169 C Calendar Gregorian 119 Islamic 119 Cancer 194 Canis Major 192 Canopus 207 Capricornus 108 Carina 207 Cassiopeia 81 Castor 194 Celestial equator 88 Centaurus 207 Cepheid variable stars 205 Cepheus 204 Ceres 149 Co-ordinate systems 63. See also specific co-ordinate systems Co-ordinates displaying with Starry Night 69 epoch 106 J2000 106 Coal Sack Dark Nebula 209 Coma Berenices 195 Comets 150 adding to Starry Night 153 Hale-Bopp 150, 151 Halley's 151 tails 151 Page numbers in italics indicate illustrations. For definitions see glossary. Index 236 Conjunctions 146 Constellations 17, 173. See also entries for individual constellations as seen from other stars 185, 186 asterisms 175 changes due to proper motion 187 defunct 181 displaying with Starry Night 27 history 177 names 175 other cultures Babylonian 179 Chinese 183 Egyptian 183 Peruvian 184 Ptolemaic 179 Sumerian 177 seasonal autumn 203 spring 194 summer 99, 206 winter 99, 190 stick figures 174 Rey's 174 Copernicus, Nicolas 35 Crux 208 Cygnus 197 D Day sidereal 74 solar 73 Daylight saving time 77 Declination 66 Delta Cephei 204 Deneb 197 E Eagle Nebula 166 Earth circumference 93 lines of latitude and longitude 64 phases 121, 122 precession 101, 103 revolution 85 rotation 33, 73 seasons 87 tilt 87 viewed from the moon 122 Eclipses 123 annular 45 eclipse season 125 lunar 127 nodes 124 solar 45 as seen from the moon 48 last total 127 observing 46 viewing with Starry Night 47 Ecliptic co-ordinate system 67, 68 Ecliptic line 35, 36, 88 Elliptical galaxy 171, 172. See also entries for individual objects Epoch 106 Epsilon Lyrae 199 Equation of time 90 Equatorial co-ordinate system 22, 65, 67 Equinox autumnal 89 vernal 66, 87 Eta Carinae Nebula 207 Exercise, Starry Night 27, 38, 50 237 Index F Field of view 22 through different instruments 23 Foucault pendulum 34 G Galactic co-ordinate system 69 Galaxies 170. See also entries for individual objects closest to Earth 201 elliptical 171, 172 spiral 169, 172 spiral arms 169 Galilei, Galileo 35, 138 Gemini 194 Globular clusters 165. See also entries for individual objects Great Red Spot 143 H Hale-Bopp 150, 151 Halleys Comet 151 Heliocentric model, proof of 35 Hercules 198 Hercules Cluster 198 Hipparchus 101 Hipparcos project 164 Horizon co-ordinate system. See Local co-ordinate system Hyades 193 J Jewel Box Open Star Cluster 208 Julian day 78 Jupiter 143 Great Red Spot 143 moons of 143 K Kepler, laws of planetary motion 132 L Lagoon Nebula 200 Latitude 65 lines of 92 Light pollution 55 displaying in Starry Night 31 moonlight 56 Light year 25 Limiting ma      !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~gnitude 24 of different instruments 24 varying in Starry Night 31 Local co-ordinate system 20 displaying with Starry Night 29 Local meridian 21 Locked rotation 123 Longitude 64 Lyra 187, 198 M Magellanic Clouds 171 Magnitude 24 limiting 24 of planets 24 Mars 140 moons 142 observing 142 retrograde motion 140 Mercury 135 Meridian, local 21 Messier objects 167. See also common names of individual Messier objects Meteor showers 152 Meteors 150 Index 238 Milky Way 168 Minor planets 149 Mintaka 191 Mithraism 102 Mizar 196 Month sidereal 118 synodic 118 Moon 115. See also entries for individual planets dark side 121 full 117 gravitational influence 103 locked rotation 123 midnight 120 motion 43, 119 new 117 orbital nodes 115, 124 parallax 116 phases 44 surface 121 viewing from the surface of 122 N Nebulae 166. See also entries for individual objects planetary 167 Neptune 145 Night vision, accessories for preserving 57 North Celestial Pole 104 North Star 80 altitude and azimuth 29 other pole stars 105 O Obliquity of the ecliptic 87 Occultations of planets 130, 146 of stars 130 Omega Centauri Globular Star Cluster 208 Omega Nebula 200 Orion 19, 23, 186, 190 Orion Nebula 191 P Parallax moon 116 stars 158 Penumbra of Earth's shadow 127 Perseus 24, 205 Perseus Double Star Cluster 206 Planetary nebulae 167. See also entries for individual objects Planets 131. See also entries for individual objects conjunctions 146 inferior cycle 133 magnitude 24 massing 147 motion 49 occultations 146 superior cycle 139 transits 134 triple conjunction 148 Pleiades 165, 193 Pluto 145 Polaris. See North Star Pollux 194 Precession 86, 101, 103 Prime Meridian 64 Proper motion 187 Ptolemy 179 239 Index R Retrograde motion 140 Rigel 191 Right ascension 65 Ring Nebula 199 S Sagittarius 199 Sagittarius Dwarf Elliptical Galaxy 201 Saturn 144 moons 145 rings 144 Scorpius 99, 202 Seasons 87 Sidereal day 74 Sidereal year 85 Sirius 192 Solar day 73 South Celestial Pole 84 South star 83 Spiral galaxy 169, 172. See also entries for individual objects Square of Pegasus 203 Star clusters 164. See also entries for individual objects globular 165 open 164 Star of Bethlehem 148 Star trails 33, 34, 80 Stars 23, 157. 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b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b.u$v$$$$$$$%% % %1%2%k%l%%%%%%%%%*&+&n&o&&&&&' ';'<'z'{'''''''~{xur]b]b]b]b]b]b]b]b]b]b] b]b]b]b]b]b]b]b]b]b U]bc U]bc U]bc U]bc]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b+'''((1(2(S(T(k(l(z({(((((((1)2)t)u)))))C*D*q*r*w*x************zup]bc]bcH]bcH]bc`]bc`]bc`]bc`]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b+*****&+'+4+5+=+>+Y+Z+++++++6,7,f,g,,,,,-->-?-t-u--þ}xsnid_Z]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bcQ]bc!----..6.7.F.G.z.{......... / /F/G///////00#0$0`0a00000þ}zwtqnkh]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b] bc(] bc(] bc(] bc(]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc'000003141E1F1i1j1{1|1}11111122=2B2C2D2222222*3+3p3q333444 4B4C444}zwtq]b]b]b]b UV]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b UV]b]b]b]b]b]b] b]b] b]b]b]b]b]b]b]b]b]b]b]b]b]b,44444455C5D55555555566B6C66666 7 7T7U77777888898k8l888889~{xu]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b,99L9M9|9}999999:::::;:C:I:J:::::;;P;Q;;;;;<<=<><Q<Z<[<\<<<<<þ}zwtqn]b]b]b]b]b]b UV]b]b] b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b UV]b]b]b]b U]bc U]bc]b]b]b]b U]bc U]bc]b]b]b]b]b+<==7=8=j=k=========>>D>E>w>x>>>>>??'?(?H?I?R?S??????@@ @!@*@|yvspm]b]b]b]b]b]b]b UV]b]b]b U]bc U]bc U]bc U]bc]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b] b]b]b]b U]bc U]bc]b]b**@+@<@=@x@y@@@@@?A@AAAAABBKBLBBBBBBBBBGGGGGHH?H@H|H}HHHHH~{xur]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]bU]b]b]b]b]b]b]b]b]b]b UV]b]b]b]b]b]b]b]b]b]b]b]b,HHH+I,IlImIIIIIJJ$J%JHJIJ`JaJJJJJ"K#KgKhKiKjKKKKKKKKK'L(LkLlLLLLL~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b]b]b]b]b,L3M4MCMDMMMMMMM*N+NhNiNNNNNNO,O-O3O4OwOxOOOOOOPBPCPPPPPQQJQKQSQTQ~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b] b]b]b]b]b]b]b]b] b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b,TQUQVQcQdQQQQQQQQQQQ2R3RlRmRqRrRRRRRRRRRRRRRSSJSKSjSkSSSSSSS~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b,SSST T T TTT+T,ThTiTTTTTUU3U4UIUYUwUxU|U}UUUUU@VAVVVVV W WW WPWQWkW~{xur]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc U]bc U]bc]b]b]b]b]b]b+kWlWWWWWXXAXBXFXGXXXYXXXXXXX Y!YFYGYkYlYvYwYYYYYYYYYYYYYZZ&Z'Z*Z+Z}zw]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]bU]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b-+ZEZFZfZgZlZZZZZZZZZZZZZZZZ [ [8[9[m[n[[[[[[[[[[[\\-\|yvspmjg]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc U]bc U]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc]b]bU]b]b]b]b]b'-\.\b\c\\\\\]]4]5]i]j]]]]]^^M^N^^^^^^^^^'_(___`_____``?`@`u`v````~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b] b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b.``aa&a'alamaaaaaa,b-btbubbbbbbbccccVcWccccc!d"dgdhddddd&e'ehe|yvsp]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b]b]b]b]b UV]b]b]b]b]b]b]b U]bc 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BRS]{}Ӱ԰^_%&ghݲ޲!"hi./st~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b] b]b]b]b]b]b]b]b]b]b] b]b] b]b UV]b UV]b UV]b]b]b,ȴɴ?@ĵŵ  &'OP׶ض)E\]|};<|yvs]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b UV]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b,12xyDEںۺ,-mnƻǻ45xy}zw]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]bU]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b-9;BCmnz{̽ͽWXZ\޾߾$%gh˿̿ OPXY~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b] b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b] b]b]b]b,YGH<=|}  ABvwQRWXdeý~{xu]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b,;<[\,-uv34xyEFnovw|}ý{vql]bc ]bc ]bc ]bc ]bc ]b]b]b]b]b]b]b]b]b]b]b]b UV]b]b]b]b]b]b U]bc U]bc]b]b]b]b]b]b]b]b]b]b U]bc U]bc]b]b]b]b)34BCGHnoþ}zvspmjgda^]b] b]b]b]b]b]b]bU]b]b]b U]bc U]bc]bc]bc]bc]bc]bc]bc]bc]bc]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc ]bc $BC IJv~WX &'CDHIcdrs}zwt]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]b]bc 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