The Celestial Sphere

Transcription

1 The Celestial Sphere Background: This unit you are going to use a star map to begin moving around the sky. Here are some simple tricks for finding your way that should save you a lot of grief. 1. Get a map! a. First things first. You need maps. To start with, you need a simple all-sky map, for use with the naked eye, that shows where to find the brightest stars and constellations as seen at your particular time, date, and latitude on Earth. b. The SkyGazer software (other CD-ROM that came with your text) can display and print a customized all-sky map for whatever time, date, latitude, and longitude you specify. Point it in the right part of the sky and then Print. Be sure that you only display useful information or the printout will be cluttered. c. You can find great star maps at or at Both are good, but I like the first for its portability. 2. Using the Map a. The map's edges represent the horizon all around you, as if you were standing in an open field and turning around in a complete circle. Compass directions should be printed around the horizon/edge. b. The center of the map represents the part of the sky directly overhead. c. A star that's plotted on the map halfway from the edge to the center, therefore, can be found about halfway up the sky -- halfway from horizontal to overhead. d. This map is drafted for specific times and dates (printed in its upper right corner). This way it avoids the distortion of the southern sky that a planisphere has to be drawn with in order to work for all times and dates. 3. Into the Night a. To read the map outdoors, bring along a dim flashlight. The best flashlight for astronomy is red, not white; red light affects your night vision less. You can rubber-band a piece of red paper or plastic over the front of the flashlight. This both dims and reddens the light. Red fingernail polish also works well.

2 b. Outdoors with your map, start by looking for only the brightest stars plotted on it. The difference between bright and faint stars in the sky is much greater than is represented on paper. In fact, if you live in a populated area where there is much light pollution (artificial skyglow), the faint stars will be completely invisible. c. Also, be aware than the constellations on an all-sky map appear much smaller than they do in real life. The star patterns you're hunting in the night are mostly big! 1. Prepare To Go Observing Outdoor observation is a big part of this class and most of what astronomers do all the time (except those who stay indoors and work at really large telescopes). So, we begin our series of outdoor observations with some tips on what to do to prepare to observe. It always helps to bring along the following: a. A sky map or star map like the ones we printed out last week. b. A printed copy of the directions (from BlackBoard), something to write on, and a pen or pencil. c. A flashlight with a red filter - you can make one from red Saran Wrap or red cellophane. In a pinch, make one from d. Clothing suitable for nighttime temperatures. e. It is also a good idea to bring another person. Standing around in the dark can be lonely. For some people it is also a bit scary to be alone in a dark and unpopulated area. Another person helps to alleviate these problems and they also learn about the stars. To have a really great observing session, you need two things: eyes that are ready for the dark and dark skies. The sensitivity of the eye depends on the intensity of the ambient light. On going from a light environment to a darker one, there is a gradual increase in sensitivity of the eye allowing dimmer objects to be seen, a mechanism known as dark adaptation. Exposure to dark allows the regeneration of overexposed receptors in our eyes. Most of the regeneration however is complete within the first 12 minutes, but it should take your eyes about 30 minutes to completely adapt to dark skies. Make sure that during this time you don t look at bright lights (unshielded flashlights, streetlights, the Moon, etc.) because this will cause the whole process to start over. 2. Find a Site Without Much Light Glow For amateur astronomers these are the best of times and the worst of times. Never has so much celestial information been available at the flip of a page or click of a mouse. But never have so many people lived under such awful skies. Many New Yorkers can follow the motion of Pluto in 3000 A.D. on a webpage, but cannot step outdoors and find the North Star through the light pollution. Here are a few pointers for choosing a spot for sky viewing: 1. There is a window with a radius of 25 (about 2 hand spans) from the zenith (straight up) where faint objects are well seen even in Albany pollution. Every effort should be made to observe within this window, or at as high an elevation as possible. 2. Exclude as much local light as you can. Observe after 11 p.m., choose the most shielded site, and if you can't observe in the shade of trees or walls, at least try to get behind some trees or a ridge that will screen the glow somewhat.

3 3. The best conditions are after rain and in high winds, as clear country air is blown over the city. 4. It helps to retire late and rise early. Even in the biggest of cities, things slow down in the dead of night. Plan ahead with sky calendars and star maps for events that will take place within one's limited hunk of sky. 5. Make sure that you choose a place you feel safe and always, always, always tell people that you are going outside to this spot. Part 1: "Poles, Dippers, and Directions" Ursa Major (Big Bear) is the third largest of the 88 officially-recognized constellations. Several stars form a smaller asterism within the constellation, variously called the "Big Dipper" (America), "Plough" (Britain), or many other names. The Big Dipper is one of the most easily recognizable groups of stars in the sky, being circumpolar (never setting below the horizon) and therefore visible in northern skies year-round. Notice whether the dipper would be full of water, or whether it's upside down in the sky. Some Native Americans associated the dipper with the colors of autumn leaves, poured out in the fall by the upside down bowl. If you can find the big dipper in the sky, you have a starting point for identifying many other stars. Learn to use it as a landmark in the sky for the following constellations. Practice tracing from the Big Dipper by finding them on your sky map--but remember that the shapes of constellations are distorted as you move toward the outer edge of the sky map, and lines that appear straight in the real sky will not look straight on the flat surface of the sky map.

4 The Pointers: The two stars forming the pouring edge of the Big Dipper's bowl (on the side away from the handle) point to Polaris, the north star, in the constellation Ursa Minor, Little Bear. Polaris is a rather faint star about five times farther away than the distance between the pointers themselves. No matter where you are in the northern hemisphere, when you face Polaris you will be facing north. Polaris marks north more accurately than a magnetic compass. The angle between your horizon and Polaris is equal to your latitude on earth (can you prove this geometrically?) Questions: 1. Facing north, find the Big Dipper and Polaris. Where is the Big Dipper compared to Polaris (east, west, etc.)? 2. Where is the Little Dipper (in terms of directions) from Big Dipper? 3. What direction is the zenith from the Little Dipper? 4. Once you can identify north-south and east-west in the sky, you will be able to read a sky map pretty easily. Now, I know that you may not be very familiar with the night sky, but we are going to use the sky map to identify two other sights in the sky. Choose two other stars. Compare their brightness to Polaris. 5. Is Polaris the brightest star in the sky? Google search your results and compare Polaris apparent brightness to other stars apparent brightness/magnitude.

5 Part 2: "Bright Things" Background: Now, we can begin by trying to find the biggest and brightest in the night sky. For Earth, this is the Sun, Moon, Planets, and man-made satellites. For this week, pick one from each of the following categories and go out and try to find each object in the night sky (record dates and what you see in each "results" section). Each category has links to websites that will help you find them on any particular day. This class is blessed with some people who know a bunch about the night sky and some folks who know nothing about the night sky. So, we are going to start at Ground Zero and build everybody up to the same point. If this gets boring, you can always check out the weekly observing challenge or ask for some more challenging stuff on the side. I would be more than happy to oblige!!! Category 1: Sun and Moon The Sun is dangerous to view (as someone has already pointed out) without good equipment and some patience, so we will save that for another week. The Moon is a great and easy target - even on windy nights, the Moon is a great view when it peeks out through clouds. For its current phase, click here (pretty cool, hunh). The trick with observing the Moon is realizing that as the moon wanes (shrinks), it becomes more visible during the day. 6. "Moon" Results: When did you look and what did you see? Category 2: Planets Planets are best described as bright stars - we are too far away to see any details like colors or rings without binoculars or telescopes. So pick two of the following and track it down using a star-map. For kicks, if you can get out more than once, check out its motion against the stars around it. - Look at the most recent Skyat a Glance here: - You are only looking at Mercury, Venus, Mars, Jupiter and Saturn you cannot see Uranus, Neptune, and Pluto without a large telescope. 7. Choose two planets from the list above. Which planets did you look for, when and what did you see? Include details about color. Category 3: Bright Guide Stars Looking due north where you'll see the Big Dipper almost directly above and just to the right of the North Star, its cup pointed down in such a way that if it were filled full of water, the water would be pouring out directly onto the ground below, which gives a celestial

6 significance to that old saying "April showers bring May flowers" because every April and May in early evening the biggest water dipper of the heavens is indeed pouring its imaginary water onto the Earth below. Now aside from the water pouring aspect of the dipper at this season, we can use its handle as a finder to find stars of late summer, which are absolutely wonderful. Going back to the Big Dipper, trace away from it above the open bowl. This line runs to Deneb, the tail of the constellation Cygnus the Swan. With wings abreast, and long neck outstretched, Cygnus flies along the milky river. One legend relates that the swan was the hero Orpheus, who enchanted all who heard him with his magic harp. Cygnus is also known as the Northern Cross, and around Christmas Eve at sunset it stands upright on the northwest horizon. Cygnus: Deneb (DEN-ebb; the tail of the swan) is a bluish-white supergiant, one of the most luminous stars known. Because it is so far away, Deneb is only the 20th brightest star in the sky. But if Deneb were as near to us as Sirius, which we see as the brightest star, then it would shine as brightly as the Moon. If Deneb were as close to us as Alpha-Centauri, we could read by its light. A small, massive object orbits a giant blue star near the center of Cygnus. Discovered in 1965 and known as Cygnus X-1, it cannot be seen with optical telescopes, but it emits intense, flickering x-rays. Most astronomers believe that Cygnus X-1 is a black hole. Matter from the bluish companion star spirals down toward the black hole, emitting x-rays as it reaches the boundary and disappears inside the black hole forever.

7 Aquila: Lyra: The bright star Altair (ALL-tare) lies in the constellation Aquila the Eagle, which was a servant of Zeus. Altair means "the flying one," and Altair flies around its axis once every 6 and a half hours. Astronomers calculate that because of this rapid rotation, it must be twice as wide at its equator as at its poles. Finally, there is bluish Vega. Vega is the second brightest star in the northern hemisphere, closely rivaling Arcturus. Vega means Swooping Eagle in Arabic. It soars almost directly overhead in summer, while the bright stars of winter nights are hidden almost directly beneath our feet. Vega is in the constellation Lyra the Harp. This is the lyre which belonged to Orpheus according to the Greeks, or to King Arthur according to English legend. Look for a small parallelogram of stars near Vega which forms the frame of the harp. 8. Choose the three stars above and observe them. What did you look for, when did you look, and what did you see? Color?

8 Category 4: Man-made Satellites There are many thousands of satellites and big pieces of destroyed satellites, rockets, and garbage orbiting the Earth. The US Air Force and NASA spend quite a bit of time tracking these pieces with radar to insure that the Shuttle, defense satellites and other low-earth orbit satellites don't get whacked by space trash. For this category, pick one of the following large satellites to track (click on the name to get a short intro on the satellite, if you wish). HST - Hubble Space Telescope ISS - International Space Station "Alpha" Numerous small, bright satellites (magnitude 3.5 or dimmer) Go to to get visibility predictions for your observing site. Enter the name of your town and you get to pick the state. I found a bunch of small towns in Columbia County where I live, so you should be able to find your site pretty easily. Once you have chosen a site, you are taken to a page where you can choose what you want predictions for. Pick a satellite and you get predictions for the next two or three days. Click on the date and you get a nice star-map. I know that the star-map is detailed, but keep in mind you are looking for a bright fast moving (almost like a plane) object following the trajectory of the solid line. The lower the line, the harder it will be to see if there are trees or building nearby. 9. What did you look for, when did you look and what did you see? 10. Even if you don t see anything, answer these two questions: a. Why are these satellites so bright? Often brighter than other stars. b. Why are these satellites moving so much faster than stars or planets across the night sky? Category 5: Iridium Flares (Extra-Credit Challenge) There are about 60 Iridium satellites in low-earth orbit. Iridiums are fast-moving satellites that are part of a failed satellite telephone system. They are slowly rotating objects that "flare" when their solar panels reflect sunlight at the Earth' surface. Very hard to see, since the flares last only seconds. Go to to get visibility predictions for your observing site. Try it, it is addictive.

9