In astronomy and navigation, the celestial sphere is an imaginary sphere of gigantic radius, centered on the Earth. All objects which can be seen in the sky can be thought of as lying on the surface of this rotating sphere.
Many ancient societies believed that the stars were equidistant from the Earth and that this sphere was a real model of the universe. This model is a useful abstraction, but not correct. Of course, we know that the objects in the sky are not on the surface of a sphere centered on the Earth, so why bother with such a abstraction? Everything we see in the sky is so very far away, that their distances are impossible to gauge just by looking at them. Since their distances are indeterminate, you only need to know the direction toward the object to locate it in the sky. In this sense, the celestial sphere model is a very practical model for mapping the sky.
The celestial sphere is divided by projecting the equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole.
As the earth rotates from west to east, the celestial sphere appears to rotate from east to west. Some stars are sufficiently near the celestial poles such that they appear to hover just above the horizon, such stars are circumpolar.
The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.