Celestial pole

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The two celestial poles are the imaginary points where the Earth's spin axis intersects the imaginary rotating sphere of "gigantic radius", called the celestial sphere.

At night ,the sky appears to drift overhead from east to west, completing a full circuit around the sky in 24 (sidereal) hours. This phenomenon is due to the spinning of the Earth on its axis. The Earth's spin axis intersects the celestial sphere at two points. These points are the celestial poles. As the Earth spins, they remain fixed in the sky, and all other points seem to rotate around them. 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, respectively).

The north celestial pole currently has nearly the same coordinates as the bright star Polaris (which is Latin for "Pole Star"). This makes Polaris useful for navigation: not only is it always above the North point of the horizon, but its altitude angle is always (nearly) equal to the observer's geographic latitude (however, Polaris can only be seen from locations in the Northern hemisphere).

The fact that the star Polaris is near the pole is purely a coincidence. It will remain a good approximation case for say 1000 years, but, because of precession of the equinoxes, considering larger time scales, Polaris is only near the pole for a small fraction of the time(the time needed for human to name it so). Actually, Polaris will remain our north star for the next 1,000 years, at which time the pole will be closer to Alrai (Delta Cephei). In about 5,500 years, the pole will have moved near the position of the star Alderamin (Alpha Cephei), and in 12,000 years, Vega (Alpha Lyrae) will become our north star.

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Sources

de:Himmelspol fr:Pôle céleste he:הקוטב השמימי nl:Pool (astronomie)

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