All of the stars and other objects that might be seen in the sky are at varying distances from us. Since even the nearest of these objects are so far away, our mind perceives them as all being at the same indeterminable distance away. For this reason and for the purpose of this amateur stargazers, the sky and all that it holds can be thought of as the celestial sphere – much like the inside of a planetarium dome. Stars are fixed points upon this sphere that all move together across the sky. Imagine the earth at the center of this sphere, and that the sphere surrounds the entire earth.
In reality, the earth spins on its axis beneath the celestial sphere. The point on the earth’s surface from which we happen to be observing circles the earth’s once each day, and we observe the celestial objects passing through our field of view, or the sky, from east to west. Our perception is that the earth is fixed, and the celestial sphere turns above us. For casual observational purposes, and for ease of explanation, the remainder of this discussion will assume the perceived motions.
© James R. Johnson, 2015.