Celestial sphere

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 poles. The celestial sphere projection is a very practical tool for positional astronomy.
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Noun
1. the apparent surface of the imaginary sphere on which celestial bodies appear to be projected
(synonym) sphere, empyrean, firmament, heavens, vault of heaven, welkin
(hypernym) surface
(part-meronym) apex, solar apex, apex of the sun's way

An imaginary sphere of infinite radius concentric with the earth, on which all celestial bodies except the earth are imagined to be projected. 1/3/73

Sphère imaginaire concentrique à la terre, de rayon infini, et sur laquelle on projette les corps célestes autres que la terre. 1/3/73

The projection of space onto a hollow sphere surrounding the Earth but centred on the observer. Only half of the celestial sphere is ever visible to an observer because the other half is masked by the surface of the Earth. Being a sphere, the easiest coordinate system to use is one of angular measurements. Several systems exist such as altitude and azimuth , right ascension and declination. The sphere itself is split up into arbitrary areas known as constellations .

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