What is Celestial sphere?
[[File:Earth within celestial sphere.gif|right|thumb|200px|The [[Earth]] rotating within a relatively small-diameter Earth-centered '''celestial sphere'''. Depicted here are stars (white), the [[ecliptic]] (red), and lines of [[right ascension]] and [[declination]] (green) of the equatorial [[coordinate system]]]] [[File:Celestial sphere.svg|right|thumb|200px|The celestial sphere is divided by the [[celestial equator]]]] In [[astronomy]] and [[navigation]], the '''celestial sphere''' is an [[imagination|imaginary]] [[sphere]] (''ball'') of a very large unspecified size. The earth and the sphere have the same centre: they are ''concentric''. They also rotate upon the same [[axis of rotation|axis]]. All objects in the [[sky]] can be thought of as [[projective geometry|projected]] (''drawn, written'') upon the celestial sphere. This imaginary sphere is a very practical tool for [[positional astronomy]]. ==Celestial hemispheres== The [[equator]] divides the earth into the [[Northern Hemisphere]] and the [[Southern Hemisphere]]. Similarly, 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. One can also locate the Celestial [[Tropic of Cancer]], Celestial [[Tropic of Capricorn]], North Celestial Pole, and South Celestial Pole. ==Sidereal time== The Earth rotates from [[west]] to [[east]] around its axis in slightly less than 24 hours.<ref>It averages once every 23 hours, 56 minutes, 4.0916 seconds.</ref> The celestial sphere and all objects on it appear to rotate from east to west around the [[celestial pole]]s in the same time. This is the [[diurnal motion]] (''daily motion''). This is why stars will rise in the east and set in the west, (unless a star is [[circumpolar star|circumpolar]]). On the next night a particular star will rise again, but with our normal clocks running a 24-hour 0-minute cycle, it will do so slightly less than 4 minutes earlier. By the following night the difference will be