polestar n : the brightest star in Ursa Minor; at the end of the handle of the little dipper; the northern axis of the earth points toward it [syn: Polaris, North Star, pole star, polar star]
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 (approximately) 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.
see 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.
see South Star Sigma Octantis is the naked-eye star closest 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 would be. At the equator it is possible to see both Polaris and the Southern Cross.
Pole stars of other planets are defined analogously: they are stars that most closely coincide with the projection of the planet's axis of rotation onto the celestial sphere. Different planets have different pole stars because their axes are oriented differently.
- Alpha Pictoris is the south pole star of Mercury, while Omicron Draconis is the north star.
- Delta Doradus is the south pole star of the Moon
- Kappa Velorum is only a couple of degrees from the south celestial pole of Mars. The top two stars in the Northern Cross, Sadr and Deneb, point to the north celestial pole of Mars.
- Delta Octantis is the south pole star of Saturn
- 15 Orionis is the south pole star of Uranus
polestar in Bulgarian: Полярна звезда (астрономия)
polestar in Catalan: Estrela polar
polestar in Spanish: Estrella polar
polestar in French: Étoile polaire
polestar in Galician: Estrela Polar
polestar in Korean: 극성 (천문학)
polestar in Dutch: Poolster
polestar in Swedish: Polstjärna
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