Tuesday, December 11, 2007


The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, "infinitely extended", intersects the imaginary rotating sphere of stars called the celestial sphere.

At night the stars appear to drift overhead from east to west, completing a full circuit around the sky in 24 (sidereal) hours. (Of course, exactly the same motion occurs during the day, except that the stars are not visible due to the sun's glare.) This apparent motion is due to the spinning of the Earth on its axis. As the Earth spins, the celestial poles remain fixed in the sky, and all other points seem to rotate around them. The north and south celestial poles are directly overhead at the North Geographic Pole and South Geographic Pole respectively.
The celestial poles are also the poles of the celestial
equatorial coordinate system, meaning they have declinations of +90 degrees and −90 degrees for the north and south celestial poles.

The celestial poles are not fixed against the background of the stars. Because of a phenomenon known as the precession of the equinoxes, the poles trace out circles on the celestial sphere, with a period of about 25,700 years. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths. Finally, over very long periods the positions of the stars themselves change, due to the stars' proper motions.