Celestial Sphere Lecture-demonstration for students preparing for the Regents Earth Science exam Developed by the staff of the RMSC Strasenburgh Planetarium Copyright ©2006 Rochester Museum & Science Center Note to teachers: This script is a composite of almost all the material that might be included in a very thorough presentation. In actual performance for your class, the order and content of the presentation will be shaped depending on the needs and interests of the teachers and students in the audience. When you arrive at the Planetarium, feel free to let your show presenter know any topics you would like particularly emphasized. Welcome to the Star Theater of your Strasenburgh Planetarium, part of the Rochester Museum & Science Center. We are seated in a circular room, 65 feet across, with 225 seats. Our seats are bolted to the floor. They're not designed to move, but they are tilted for comfort, each row at a different angle to give you the best possible view from your location. Above us, a dome...a ceiling in the form of a hemisphere. That dome serves as the screen for projected images of the sun, moon, planets and stars. Great effort and expense was devoted to making the Planetarium dome smooth and even in color so it would seem to disappear when the lights fade. Fade to blues, then spots on Zeiss The big machine in the center of the room is the projector that puts the sun, moon, planets and stars in the planetarium sky. Our star projector was custom-built for us in what was then called West Germany by the Carl Zeiss company and was installed in 1968. Each of the blue star spheres projects the stars for one half of the sky. Together, the two star spheres accurately re-create Celestial Sphere tape version 5-14-05 every star that a person with 20/20 vision could see under perfect viewing conditions from anywhere on the Earth. In the black cages near the center of the Zeiss instrument are other projectors, responsible for the sun, moon and the five planets you can see without a telescope. CZ spots fade OFF The basic purpose of a planetarium is to re-create a sky that's better than reality. We can see changes that would take hours, days or months to notice outdoors. And we can see how the sky would change if we were to travel thousands of miles north or south over the Earth in only a few seconds. THE DOME AND THE SKY When you go outside and look up, it's not hard to imagine that the sky is a just a dome. In ancient times, people thought it really was a dome, and that it continued beneath us to make a complete sphere, the celestial sphere. Today, it's still handy to think of the celestial sphere when we're finding things and understanding how they move in our sky. OPERATOR: Green arrow The highest point on the dome represents the point directly overhead in the sky, the zenith. The base of the dome represents the line where earth and sky meet, the horizon. The four directions we know from maps and compasses are marked around the base of the dome with green letters. South is directly in front of us; behind us is north; 2 Celestial Sphere tape version 5-14-05 to our right is west; to our left, east. THE SUN'S PATH ON THE MARCH EQUINOX And, to complete the scene, there's the sun, where it appears on the March equinox, usually March 20th1, also known as the vernal equinox, or, in the United States, the first day of spring. Early in the morning, the sun is low in the eastern sky. Now we'll put time into fast forward, compressing hours into seconds. OPERATOR: diurnal motion "+" [:10] The sun begins to go up—or so it appears. Of course we know that we live on a round planet, Earth, which is rotating so slowly and smoothly that we don't feel the motion, so things in the sky appear to rise and set. OPERATOR: Zeiss meridian ON, stop diurnal motion at solar noon In the middle of the day, the sun crosses an imaginary line called the meridian, from Latin words meaning "middle of the day." 1 Dates of March equinox, EST: 2004 Mar 20 2005 Mar. 20 2006 Mar. 20 2007 Mar. 21 UT, Mar. 20 EST 2008 Mar. 20 2009 Mar. 20 3 Celestial Sphere tape version 5-14-05 The meridian is the line that goes from due south, over your head and through your zenith, to due north. In the morning, the sun has not yet reached the meridian. The Latin words for “before the meridian” are “ante meridiem” or “a.m.” When the sun crosses the meridian it reaches the highest point in its path through our sky. This is noon, sundial time. At that moment, the sun is due south. When the sun crosses the meridian, just how high in our sky is it? We can measure that, in degrees of angle above the horizon. That angle is called the altitude. The altitude of the horizon is zero degrees; the altitude of the zenith is 90 degrees. On the day of the March equinox, when the sun crosses the meridian in Rochester's sky, its altitude is about 47 degrees. After the sun passes the meridian, it sinks toward the horizon and sets. OPERATOR: Resume diurnal motion In Latin, these are the post-meridiem, or p.m. hours. [:07] When the sun sets, the sky darkens and the stars appear. [:30] THE APPARENT MOTION OF THE STARS OPERATOR: As the sky darkens, bring on stars, dim the meridian. Turn the sun OFF after it sets. Thousands of years ago, before we had any way of knowing that the stars are faraway suns in space, we used our powerful human imagination to invent figures of people and animals among 4 Celestial Sphere tape version 5-14-05 the stars. For example, in the evening skies of springtime, we can see a star group that ancient people named Leo, the lion. OPERATOR: Add Leo outline The name Leo is still used by astronomers today. It's an easy pattern to find, once you learn it, so it makes a handy landmark for observing the passage of time. North of Leo, a famous star group, the Big Dipper. OPERATOR: Elmer's Big Dipper [:03] The last two stars in the cup point to the North Star, also known as Polaris or the pole star. OPERATOR: Green arrow from Dipper to Polaris, Arcturus, Spica The Big Dipper’s handle points to: a very bright orange star called Arcturus, in fact the fourth brightest star in the entire night sky… …and a bluish star called Spica, the 16th brightest. We still have time in fast forward. As the hours of the night go by, you can see old stars disappearing behind the western horizon while new stars pop into view above the eastern horizon. As you watch a while longer you realize that the stars are actually rising, climbing to their individual high points when they cross the meridian, and setting, just as the sun does during the daytime. As the constellations go across the sky, they keep the same shapes. So, for the purpose of finding our way around the sky, we 5 Celestial Sphere tape version 5-14-05 can imagine the stars just as ancient people did -- as lights attached to the inside of the celestial sphere. We're seeing stars rising and setting, but different stars take different amounts of time to go across the sky. Stars down near the southern horizon travel in low, short arcs from their rising to their setting points. OPERATOR: point to such a star, possibly Fomalhaut Stars that are farther north travel in long, high arcs across the sky. OPERATOR: point to such a star, possibly Capella Finally, in the northernmost part of the sky, those arcs become complete circles. Stars in that northern part of the sky— for example, the stars of the Big Dipper, never go below the horizon at all. They move like passengers on a Ferris wheel at an amusement park, on each trip around the circle dipping near the ground but never touching it. Those are the circumpolar stars, so called because they go around the north celestial pole, which we will see in a moment. Actually, all the stars make complete circles in our sky each time the earth rotates. If a star sets, it just finishes its circle below the horizon, where we can't see it. If there were another dome beneath our feet, and if the floor of the planetarium were transparent we could look down into the lower half of the celestial sphere and see stars traveling along the parts of their circular paths that are hidden from us now. Maybe planetariums of the future will have that feature.2 2 The Futuroscope amusement park in Potiers, France, near Paris, has a theater with a second hemisphere under the audience’s feet, but it’s not a planetarium. The double hemisphere is used for their own film “Voyageurs du Ciel et de la Mer” (Voyagers of the Sky and Sea). 6 Celestial Sphere tape version 5-14-05 Look again at the northern sky, and find a star that does not move. [:10] That star, the North Star or Polaris, the pole star, just happens to be near the point around which all the surrounding stars are circling: the north celestial pole.