celestial sphere

An imaginary sphere surrounding the Earth, used as a reference frame to specify the positions of celestial objects on the sky. Its N and S poles lie over those of Earth, and its equator is the projection of the terrestrial Equator.

Projected, from their corresponding geographic equivalents, are the celestial equator and the celestial poles.

The celestial sphere can be used geocentrically and topocentrically.

In the Aristotelic and Ptolemaic models, the celestial sphere was imagined as a physical reality rather than a geometrical projection (see heavenly sphere).

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 around its axis once every 23 hours 56 minutes, the celestial sphere and all objects on it appears to rotate from east to west around the celestial poles in the same time.

The reason for this apparent misadjustment of our clocks is that the Sun is not standing still on the celestial sphere, as the stars do, but moves about 1° per day eastwards over a great circle known as the ecliptic (which is 360° or a full circle in one year, the annual motion of the Sun). As an angle of 1° corresponds to 4 minutes in time (360° = 24 hours), we need therefore 4 extra minutes of diurnal motion to see the Sun back on (for example) the meridian again, making the duration of one rotation just 24 hours exactly (on the average, ignoring small seasonal variations, see equation of time)

Normal clocks therefore indicate solar time.

A celestial sphere can also refer to a physical model of the celestial sphere. Also known as a star globe, this sort of celestial sphere will indicate which constellations are visible at a given time and place.