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It is an imaginary sphere on whose surface the stars visible to the naked eye are projected.
The concept of the celestial sphere was introduced in the epochs of ancient astronomy and can be perfectly understood when one is willing to observe, in a serene night, the sky in a place with the free horizon. Then it seems that the stars are all on a spherical surface of infinite radius that, with the passing of the hours, turns from East to West.
It is obviously a mere appearance: in reality the celestial bodies occupy different distances from the observer; while the movement of the celestial sphere is none other than that of the Earth, which revolves around its own axis from west to east.
A complete rotation of the celestial sphere, that is, a return of the same star to the same point, is done in 23h 56m 04s (sidereal day). An observer located at one of the two poles would see the celestial sphere rotate around the vertical axis, which in this case coincides with the polar; an observer located at any other point on Earth sees it rotate around the polar axis, which is inclined with respect to the horizon at an angle equivalent to the latitude of the place.
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