Celestial sphere

In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.

Earth rotating within a relatively small-radius geocentric celestial sphere. Shown here are stars (white), the ecliptic (red, the circumscription of the Sun's apparent annual track), and the lines of right ascension and circles of declination (cyan) of the equatorial coordinate system.

The celestial sphere is a conceptual tool used in spherical astronomy to specify the position of an object in the sky without consideration of its linear distance from the observer. The celestial equator divides the celestial sphere into northern and southern hemispheres.

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