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Pole star
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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 orientation of the Earth's axis, which moves over time (see precession of the equinoxes).
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Encyclopedia
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 orientation of the Earth's axis, which moves over time (see precession of the equinoxes). 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.
North pole
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.
South pole
s 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.
Other planets
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.
The location of the celestial poles of the planets is shown in the following table:
The declination of Uranus' north celestial pole is negative as it spins 'backwards'. Pluto's north celestial pole would likewise be below the Earth's celestial equator were it not for the large angle of its orbit with respect to the ecliptic. Venus should have a negative declination also though the reference material omitted this.
Some asteroids have more than one axis of rotation, possible due to impacts with other bodies. These asteroids tumble in space and have no pole stars. It is possible that a planet could be similarly affected, though unlikely as the much greater mass of a planet and its (usually) spherical shape make it much more unlikely for an impact to create a second axis of rotation.
In mythology
- In Hindu mythology, the Pole star is called Dhruva, an ardent devotee of the god Vishnu, who was blessed to be in a high position in the sky.
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