Celestial Sphere

The celestial sphere is an imaginary sphere surrounding the Earth, on which the stars seem to be placed and which seems to rotate from east to west. Ancient astronomers believed this actually to be the case, with the stars as crystal studs, or distant holes through which fire is observed. Usually the Earth is put as its center, forming the so-called geocentric celestial sphere, the basis for the standard coordinate system in astronomy.

The axis of rotation of the Earth extends to meet this sphere in the north and south celestial poles. The celestial equator (or equinoctial) is the great circle midway between the poles. The yearly path of the Sun across the celestial sphere is a great circle called the ecliptic. The points in the ecliptic furthest north and south of the celestial equator are called the summer and winter solstices. The point where the sun crosses the equator moving north or south are called the vernal or autumnal equinoxes. Because of the Earth's rotation, the celestial sphere appears to rotate once every sidereal day. This is about four minutes shorter than the mean solar day because of the Sun's motion.


1. cones expanse   2. cutball area  3. width and volume formula

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