In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected, from their corresponding terran equivalents, are the celestial equator and the celestial poles.
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. 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 tool for positional astronomy.
As the Earth rotates on its axis, the objects on the celestial sphere will appear to rotate around the celestial poles every 24 hours, this is diurnal motion. For example the Sun will typically appear to rise in the east and set in the west, as will the stars, planets and moon. On each subsequent night, a given star will rise ~4 minutes earlier than it rose the previous night. Superimposed on diurnal motion is; intrinsic motion as the objects change their relative positions, with respect to Earth. For example, over the course of a year the Sun, relative to the background stars, will follow a bisecting great circle (known as the ecliptic).
See also: prograde and retrograde motion
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.
See also: Geocentric universe