This animation is a simple model of the apparent motion of the stars in the night sky. Ancient people described the motions as if ...
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The sun rises and the sun sets with a rhythm which repeats daily with the sun moving east to west across the sky. While the sun is down the stars form patterns in the sky which also generally move east to west across the sky. The earliest history of astronomy is the story of mankind’s efforts to develop an explanation for the appearance of the heavens as well as a deeper understanding of the nature of the universe.
Consider watching a clear night sky from somewhere in the northern hemisphere, north of the tropics. At first glance the stars seem pretty much fixed in their positions in the sky. They seem to form patterns and shapes in the sky which we call constellations. With some patience (say 10 minutes), we should be able to see the stars near the eastern horizon have risen in the sky by a couple of degrees (corresponding to the width of your pointer finger held at arm’s length) and the stars to the west have dropped about the same amount. The constellations themselves do not seem to have changed in size or shape. As we continue to star gaze, this motion of the stars continues. Towards the southern horizon stars rise and set without getting very high in the sky. Towards the north is a star that does not appear to move at all (let’s call it Polaris) and around it is a circular patch that doesn’t set but rather moves in a circle about Polaris. Throughout the entire night the size and shape of the constellations remain unchanged.
The ancients observed these motions of the stars in the sky, and described the motions as if the stars were all attached to a vast globe centered about the earth. This globe, which we refer to as the Celestial Sphere, rotates about once a day on an axis that is lined up with Polaris. This simple model nicely describes the apparent motion of the stars in the night sky.
If we go out on a clear night about a month later we see the same process repeated, with all the same constellations. However, the constellations that were just above the eastern horizon at sunset last month are now a little higher in the sky, by about 3 times the width of a fist held at arm’s length. The sun's position in the celestial sphere can be determined by noting the position of the constellations during the night and extrapolating to the daytime sky when the constellations are no longer visible. The sun apparently moves eastward through the celestial sphere at a rate of close to one degree per day, making one lap of the zodiacal constellations each year. In fact, the ancient Babylonians defined the angular unit of the degree in terms of how far the sun moved through the celestial sphere in one day.
To learn about early methods of navigation including the use of stars, check out, check out Ways of Navigation.
To learn about the beginnings of the scientific revolution that took us out of the center of the universe, check out Galileo: Sun-Centered System.
- How would you expect observations of the night sky to change if you were watching from a different vantage point on the Earth, say on the equator, south of the tropics or at either pole?
- Suppose the Earth were flat, and the stars really were attached to a sphere that spun around the Earth. Think about travelling north until you were directly under Polaris. Would you expect the same motions of the stars that someone at the north pole of a spherical Earth would see?
- Suppose we lived on a flat Earth and the Celestial Sphere had a diameter that was just a little bigger than our flat Earth. What would happen to the appearance of the constellations as you travelled towards the edge of the Earth? Think about both the constellations near the edge you are travelling towards as well as those constellations on the opposite side of the Celestial Sphere.
- Consider watching the shapes of the constellations over the course of a year as the Earth orbits the Sun. What might we expect to happen to the size of the constellations? One of the reasons Copernicus’s contemporaries discarded his Sun-centered cosmos is that the apparent shapes and sizes of the constellations did not change over the course of a year. What properties would our Celestial Sphere model need to accommodate both a moving Earth and no observable changes in constellations?
Learn more at Animations for Physics and Astronomy.