Pole star
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45 minute exposure of stars appearing to circle the South Celestial pole |
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.
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)
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.
See also
*
Precession of the equinoxes*
Astronomy*
North Star (
Polaris Borealis)
*
South Star (
Polaris Australis)
*
Lode Star*
Celestial navigationExternal Links
Star trails around Polaris
