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( Polestar)
A pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent position is close to one of the celestial poles, and which lies directly overhead when viewed from the Earth's North Pole or South Pole. There are potentially both northern and southern pole stars, but whether there is either depends on the current stellar configuration. The term the pole star usually refers to the star Polaris (colloquially referred to as the "north star") which is the current northern pole star. Pole stars change over time because stars exhibit a slow continuous drift with respect to the Earth's axis. The primary reason for this is the precession of the Earth's rotational axis that causes its orientation to change over time. If the stars were fixed in space, precession would cause the celestial poles to trace out imaginary circles on the celestial sphere approximately once every 26,000 years, passing close to different stars at different times. However, the stars themselves exhibit motion relative to each other, and this so-called proper motion is another cause of the apparent drift of a pole star. Pole stars are often used in celestial navigation. While other stars' positions change throughout the night, the pole stars' position in the sky essentially do not. Therefore, they are a dependable indicator of the direction toward the respective geographic pole, and their angle of elevation can also be used to determine latitude. At the present time, Polaris is the pole star in the northern direction. Its mean position (taking account of precession and proper motion) will reach a maximum declination of +89°32'23", so 1657" or 0.4603° from the celestial north pole, in February 2102. (Its current declination is +89°15'50.8".) Its maximum apparent declination (taking account of nutation and aberration) will be +89°32'50.62", so 1629" or 0.4526° from the celestial north pole, on 24 March 2100.[1]
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