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In Astronomy and Navigation , the celestial sphere is an Imaginary Rotating Sphere of "gigantic Radius ", Concentric and Coaxial with the Earth . All objects in the Sky can be thought of as lying upon the sphere. Projected, from their corresponding Geographic equivalents, are the Celestial Equator and the Celestial Pole s. The celestial sphere projection is a very practical tool for Positional Astronomy .

In the Aristotelic and Ptolemaic models, this sphere was imagined as a physical reality rather than a geometrical projection, see Heavenly Sphere .

As the Earth rotates on its axis, the objects on the celestial sphere will appear to rotate around the Celestial Pole s about 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 Minute s 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 ).

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 . If a star is sufficiently near the celestial pole that is above the Horizon , the star is also always above the horizon, encircling the pole; such stars are Circumpolar .

The directions toward various objects in the sky can be quantified by constructing a Celestial Coordinate System .

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.


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