The imaginary sphere of infinite radius within which celestial bodies appear to lie. The earth, and the observer, are visualized as being at the centre of the sphere and the sphere as rotating once every sidereal day. The sphere is used to describe the position of celestial bodies with respect to the earth.
Celestial sphere.
An imaginary sphere of indefinite size, used as a background for assigning positional coordinates to celestial objects (see diagram). The sphere may be centred on the Earth, the observer, or any other point which acts as the origin of the chosen system of coordinates. As seen from Earth, the celestial sphere appears to rotate about the axis joining the celestial poles every 23 h 56 m 04 s (a sidereal day), actually a result of the axial spin of the Earth. Two important circles on the celestial sphere are the celestial equator and the ecliptic. The angle between them is about 23°.4, and is known as the obliquity of the ecliptic. The celestial equator and the ecliptic intersect at the vernal and autumnal equinoxes. The positions of the celestial poles, and hence of the celestial equator, are gradually moving on the celestial sphere because of a slow wobbling of the Earth in space termed precession. See also celestial coordinates.
An imaginary sphere surrounding the Earth, on which the celestial bodies seem to lie. The positions of bodies such as stars, planets, and galaxies are specified by their coordinates on the celestial sphere. The equivalents of latitude and longitude on the celestial sphere are called declination and right ascension (which is measured in hours from 0 to 24). The celestial poles lie directly above the Earth's poles, and the celestial equator lies over the Earth's Equator. The celestial sphere appears to rotate once around the Earth each day, actually a result of the rotation of the Earth on its axis.
The main features of the celestial sphere. Declination runs from 0° at the celestial equator to 90° at the celestial poles. Right ascension is measured in hours eastwards from the vernal equinox, one hour corresponding to 15° of longitude.
Credit: Research Machines plc