Pole star
From Wikipedia, the free encyclopedia
A pole star is a visible star that is approximately aligned with the Earth's axis of rotation; that is, a star that lies in the direction pointed to by one of Earth's poles. There are potentially both north and south 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 but distinct 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 position of a pole star to trace out an imaginary circle on the celestial sphere approximately once every 26,000 years. However, the stars themselves exhibit motion relative to each other (including the sun), 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 does not. Therefore, it is a dependable indicator of the direction north.
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[edit] North pole
- Further information: North Star
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. (ref: Jean Meeus, Mathematical Astronomy Morsels Ch.50; Willmann-Bell 1997)
[edit] South pole
- Further information: South Star
Sigma Octantis is the closest star to the south celestial pole, but it is too faint to serve as a useful pole star. The Southern Cross constellation functions as an approximate southern pole constellation, by pointing to where a southern pole star should be. At the equator it is possible to see both Polaris and the Southern Cross.
