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When you
look at the sky on a clear, dark night you might think that you can
see millions of stars. In reality, the human eye can only detect about
6000 stars over the entire sky. However, for those of us in the northern
hemisphere we can only see the stars that are above our horizon, which
means that we can only see about 3000 stars. The earth rotates from west to east once every 24 hours, which is why we have day and night, so the stars rise in the east and set in the west, as do the Sun and the Moon. This daily, or diurnal, motion of the stars  is
very apparent in time exposure photographs. Many ancient societies believed
the Earth to be the center of the universe. They also imagined the stars
to be attached to the surface of a huge sphere centered on the Earth.
This imaginary sphere, called the celestial sphere, is quite a useful
concept. The stars are, in actuality, scattered at various distances from the Earth even though they all appear to be about the same distance away from us. Several of the brightest stars that are visible to the naked eye are in the range of 10 to 1000 light years away. These are very large distances indeed, so far in fact that the stars appear to be fixed to a spherical backdrop. In fact most of our understanding of the motions in the sky began with ancient astronomers who viewed the sky as if the Earth was at the center of the universe. Although we know this not to be the truth, it is still very useful to use this viewpoint when studying the night sky. Lets picture the Earth at the center of a huge sphere called the celestial sphere. Lets project some of Earth's key geographic features outward into space to provide the celestial sphere with some features as well to establish directions and our bearings. Lets extend the following Earth's features outward:
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