In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the celestial equator and the celestial poles. The celestial sphere is a very practical tool for positional astronomy.
In the Aristotelic and Ptolemaic models, the celestial sphere was imagined as a physical reality rather than a geometrical projection (see Celestial spheres).
The celestial sphere can be used geocentrically and topocentrically. The former means that it is centered upon an imaginary observer in the center of the Earth, and no parallax effects need to be taken into account. In the latter case it is centered upon an observer on the surface of the Earth and then horizontal parallax cannot always be ignored; especially for the Moon.
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. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.
As the Earth rotates from west to east around its axis once every 24 hours, the celestial sphere and all objects on it appear to rotate from east to west around the celestial poles in the same time. This is the diurnal motion. Therefore stars will rise in the east, culminate on the north-south line (meridian) and set in the west, (unless a star is circumpolar). On the next night a particular star will rise again, but with our normal clocks running a 24 hour 0 minutes cycle, it will do so 4 minutes earlier. By the following night the difference will be 8 minutes, and so forth with every following night (or day).
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