star can be described by its altitude angle above the horizon and its azimuth angle Q14: Make another careful sketch, again noting the time and date.
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4 ASTRONOMY 113Laboratory Introduction Astronomy 113 is a hands -on tour of the visible universe through computer simulated and experimental exploration. During the 14 lab sessions, we will encounter objects located in our own solar system, stars filling the Milky Way, and objects located much further away in the far reaches of space. Astronomy is an observational science, as opposed to most of the rest of physics, which is experimental in nature. Astronomers cannot create a star in the lab and study it, walk around it, change it, or explode it. Astronomers can only observe the sky as it is, and from their observations deduce models of the universe and its contents. They cannot ever r epeat the same experiment twice with exactly the same parameters and conditions. Remember this as the universe is laid out before you in Astronomy 113 Ð the story always begins with only points of light in the sky. From this perspective, our understandin g of the universe is truly one of the greatest intellectual challenges and achievements of mankind. The exploration of the universe is also a lot of fun, an experience that is largely missed sitting in a lecture hall or doing homework. The primary goal of theses labs is to bring you closer to the reality of astronomical research, and in so doing to the experience of science. Of course, this would be best done at night with real telescopes, but the vagaries of Madison weather make this impractical with lar ge classes. Fortunately, computer simulation software does remarkably well in recreating the experience of working at telescopes, including some of the largest in the world. That having been said, the lab does include voluntary night viewing sessions at Washburn observatory giving you a taste of the wonders of the night sky. These labs are designed to provide you with opportunities to explore and discover. Always remember that this is your exploration Ð different students will follow different paths, all of which can lead to interesting results. DonÕt be afraid to try things out and to experiment. Trial and error is a valid way to explore a new environment and you cannot break the software (if you do, itÕs not your fault Ð just get your instructor to restart the software and you will be good to go again). The organizational details for this lab can be found in the lab syllabus and on the web: http://www.a stro.wisc.edu/~sstanimi/ ast_113_fall11.html Be inquisitive and collaborate You are always welcome to give us feedback and suggestions about this lab. And donÕt ever hesitate to ask if you have a question: You are the best judge of what you have and have not understood. The goal of this lab is for you to learn. The best way to reach that goal is to ask questions Ð either to your lab
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5 partners or to your instructor. You will find that asking questions is often the best way to approach a problem, and every scientific endeavor; every research project (in Astronomy and all other sciences) begins with a question. The labs in Astronomy 113 are best done in groups. For that reason, you will team up with at least one other lab member (but no more than 3) to go through the lab together. This is another good analogy to real world astronomical research: Most proj ects in astronomy require collaborations between people with different expertise. You will find that you reach a much deeper level of understanding of something after trying to explain it to somebody else and after you have discussed a question with somebody else. As you go through the lab, discuss the questions in the manual and try to explain your answers to these questions to your lab partner and to listen to their explanation. The only time collaboration in the lab is not allowed is, of course, during the quizzes. Computers: As mentioned above, most of this lab is computer based. Sterling 3517 provides a number of iMac computers. The labs are very easy to operate and the software is very user friendly. Your instructor will help you with all compute r related questions Ð the computers are only a tool to bring you closer to real astronomical observations. Your time in the lab is best spent exploring the questions and tasks in the lab manual, rather than fighting with the computer. Again, if you have trouble with the operation of the computers, ask your instructor or lab partners Ð the earlier the better (to get you going again so you can finish the lab in time, or even ahead of time). Final thought: Remember: This lab is a chance for you to explore a nd to get a taste of what astronomical research is like. Use all the resources you have available Ð the regular lab hours, the open labs/office hours, the web, E -mail, our mail boxes, your home computers, your text books and notes from Astronomy 100 or 103, and whatever else you can think of. Most importantly, though: Have fun and enjoy the hands on tour through our universe!
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8 Before You Co me to Class Read the lab completely. Your time in the lab is best used observing the “sky”, not reading this manual. Bring to class this lab manual, your lab book, a pencil or erasable pen, a straight edge, and a scientific calculator. A pre -lab is due at the beginning of the second lab session (i.e., the second week of this lab). Schedule This lab is designed to be completed in three lab sessions. You should be well into if not completed Section 4 in the first lab session, through at least Sec tion 6 in the second lab section, and then complete the lab in the third lab session.
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9 Section 1: Sunset It is dusk, and the Sun has just set. You are standing in a meadow, looking toward the northern horizon. Above you is the sky (no sta rs yet!) and below you is the ground. Curiously, there are letters on the horizon indicating which direction you are facing (north, south, east and west). You turn your head to look in different directions: ¥ Move the horizontal scrollbar with the left an d right arrows, or by dragging the scrollbar tab. (Note that when you grab the scrollbar tab, a compass appears to show the present direction.) You can also maneuver on the sky with the arrows on the keyboard. ¥ Return to looking north. As the sky gets darker, more and more stars appear in the sky: ¥ Turn on the stars. (Display menu to Stars; select Show Stars, then click OK) Now you tilt your head back to look at the stars overhead: ¥ Move the vertical scrollbar with the up and down arrows, and by g rabbing the scroll bar. When you grab the scroll bar, an indicator appears, showing the angle above the horizon at which you are looking. ¥ Find the point in the sky marked “zenith.” The zenith is the point directly overhead. Of course, you will never see it marked in the real night sky! ¥ Lower your head (move the vertical scrollbar) so that the “North” is just above the bottom of the screen. Section 2: Figures in the Sky At first glance, the stars appear to be scattered at random in the sky. But i t is the nature of humans to organize, and archeological records show that all civilizations have seen patterns in the stars, or constellations. The stars were thought to be in the realm of the gods, and often the constellations were linked to religion and myth. In Western cultures the constellations typically derive from Greek and Roman mythology, such as Aries, Leo, Andromeda, Orion, Hercules and Gemini. These patterns gave an organization to the sky that was essential for its study. Indeed, some people b elieve that many myths were made up for the sole purpose of remembering star patterns. Today, while few give any spiritual significance to the constellations, every culture still uses constellations to guide their way through the sky. Since the constellat ions are simply mnemonics, there is no “right” way to group stars. Different civilizations have created different sets of constellations from the same night sky. However, there are certain groupings of stars that are so distinctive that every civilization has grouped them together — although not always representing the same thing. One such group of stars is the one we call the Big Dipper, also seen as a Starry Plough in England, as a Wagon in Europe and Israel, and as the Government in ancient China! Find the Big Dipper in the Voyager sky. Once you have found it: ¥ Turn on the constellation lines and labels. (Display menu to Constellations; click the tab Lines and Labels, then select Show Constellation Lines and Show Constellation Labels)
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10 ¥ Turn on the co nstellation figures. (While still in the Constellations panel, click on the tab Figures, then select Show Mythological Figures and click OK) The Big Dipper is actually the body and tail of Ursa Major, the Large Bear. ¥ Use the scrollbars to wander around the sky. How many constellations do you recognize? In ancient times the 48 constellations catalogued by Ptolemy were widely accepted, but as astronomy became more precise (and as the skies of the southern hemisphere were included), many more constellations were added. There are now 88 constellations, as established by the International Astronomical Union in 1930. At that time boundaries were established for each constellation, so every star in the sky falls in exactly one constellation. If you wish to se e the boundaries, turn on Constellation Boundaries (Display menu to Constellations; click the tab Boundaries and Regions, select Show Boundaries and click OK). Be sure to turn the boundaries off again when youÕre finished! Section 3: Coordinates in the Sky Constellations are a reasonable system of organization for “naked eye” observations, but suppose that you wanted to direct someone to a faint comet that you just discovered in your telescope. It would hardly do to tell her to point her telescope just to the left of the nose of Pegasus! Astronomers have developed several coordinate systems to solve this problem. We’ll introduce you to one in this lab — the altitude-azimuth coordinate system Altitude and azimuth are just more sophisticated versions of your natural inclinations. The position of a star can be described by its altitude angle above the horizon and its azimuth angle around the horizon (analogous to a compa ss direction, like “southeast”). Altitude is measured from 0û at the horizon to 90û at the zenith. Azimuth is measured around the horizon from north to east. So north has an azimuth of 0û, east has an azimuth of 90û, south has an azimuth of 180û, and west has an azimuth of 270û. ¥ Move the horizontal scrollbar to look north again, with the “North” just above the bottom of the screen. ¥ Turn off the Constellation Figures. (Display menu to Constellations to Figures, then deselect Show Mythological Figures a nd click OK) ¥ Open the Coordinates Panel. (Window menu to Coordinates Panel) In addition to several other values in the Coordinates Panel, you will see values showing the azimuth and altitude of the cursor. Note that both are measured in degrees, arcminutes, and arcseconds. There are 60 arcminutes in 1 degree, and 60 arcseconds in 1 arcminute. The azimuth and altitude shown in the second column (to the right of the ! symbol) are the change in coordinates from the location of the cursor when you last clicked the mouse. To get a feeling for the alt -az system, move the cursor around the sky and watch the azimuth and altitude values. For example, move along the horizon from due north to the east and to the west. How do the altitude and azimuth change? Now move from the “North” straight up. How do the altitude and azimuth change? Notice that the azimuth changes abruptly by 180û at the zenith. Q1: What are the altitude and azimuth angles of the end star in the handle of the Big Dipper?
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11 What constellation is nearest to azimuth 90û, altitude 0û? What constellation is nearest to an altitude of 90û? ¥ Turn on the coordinate grid. (Display menu to Coordinate Grids, then select Show Grid Lines and click OK) The grid makes it evident that every star has its own unique altitude and azimuth. Note that the “pole” of this grid is the zenith. While the “alt -az” coordinate system is very intuitive and has many valuable applicat ions, it does have two notable weaknesses. First, the azimuth and altitude of every star are always changing with the passage of time. Second, the altitude and azimuth of a star at any given time depend on the location of the observer on the Earth. So it is a bit of a trick to tell your friend the altitude and azimuth of your faint comet if she happens to be elsewhere in the world. Later in the course we will introduce you to a celestial coordinate system in which a star’s position is always the same (almost!). Section 4: The Polaris Experiment Polaris, the North Star, has long shown the way to travelers. At sea the only “landmarks” are celestial. It was Polaris that guided sailors around the globe, and not only by showing the way North. Using a sextant a European captain sailing to the New World could also accurately find the ship’s latitude on any clear night by observing Polaris. While it is seldom still needed today, Polaris remains ever present to guide the lost or the adventurous. To find Polaris ¥ Turn off the coordinate grid. (Display menu to Coordinate Grids, deselect Show Grid Lines, then click OK) Look north again, with the “North” just above the bottom of the screen. People often have the misconception that Polaris is a particularly br ight star. It is not, but it is easy to find nonetheless. The trick is to first find the Big Dipper. Then locate the two stars in the bowl that are farthest from the handle. These are the “Pointer Stars”, Dubhe and Merak. Now imagine a line beginning at M erak (the one at the bottom of the bowl) and passing through Dubhe. This line passes through Polaris at about five times the distance between the Pointer Stars. There are no bright stars near Polaris, so it is hard to miss once the Pointer Stars show you the way. To find out if you have correctly identified Polaris, put the cursor on the star and click. A window will pop up with the star’s name and information about it. If the star is not Polaris, keep looking. (Put the window away by clicking on the first circular button in its upper left corner.) Q2: In which constellation is Polaris? Make a sketch of the stars in the constellation and identify Polaris. Include the Big Dipper in your sketch. The brightest stars in each constellation are named by a le tter of the Greek alphabet and the Latin genitive form of its constellation name. The brightest one gets Alpha, the next brightest gets Beta, then Gamma, Delta, Epsilon, and so on. For example, Polaris is Alpha Ursae Minoris. The brightest star in the sk y is Sirius, the Dog Star, which is also known as Alpha Canis Majoris. (The constellation Canis Major is the Large Dog.) There are many more stars than Greek letters, however, so this system only applies to the 24 brightest stars in any constellation. Now it is time for your first measurements — in particular, you will measure the altitude of Polaris from a number of different locations on the Earth. You can measure altitude by putting the cursor on Polaris and looking at the Coordinates Panel.
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