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A pole star is a visible star, preferably 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. A similar concept also applies to other planets than the Earth. In practice, the term pole star usually refers to Polaris, which is the current northern pole star, also known as the North Star.
The south celestial pole currently lacks a bright star like Polaris to mark its position. At present, the naked-eye star nearest to this imaginary point is the faint Sigma Octantis, which is sometimes known as the South Star.
While other stars' apparent positions in the sky change throughout the night, as they appear to rotate around the celestial poles, pole stars' apparent positions remain virtually fixed. This makes them especially useful in celestial navigation: they are a dependable indicator of the direction toward the respective geographic pole although not exact; they are virtually fixed, and their angle of elevation can also be used to determine latitude.
The identity of the pole stars gradually changes over time because the celestial poles exhibit a slow continuous drift through the star field. The primary reason for this is the precession of the Earth's rotational axis, which 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 also exhibit proper motion, and this motion is another cause of the apparent drift of pole stars.
The current northernmost star is a rather unremarkable 19th-magnitude star called USNOA2 1725-00522696. However, the closest bright star to the pole is Polaris. At magnitude 1.97 (variable), it is the brightest star in the Ursa Minor constellation (at the end of the "handle" of the "Little Dipper" asterism). Its current (October 2012) declination is +89°19'8" (as per epoch J2000 it was +89°15'51.2"). Therefore it always appears due north in the sky to a precision better than one degree, and the angle it makes with respect to the horizon is equal to the latitude of the observer. It is consequently known as Polaris (from Latin stella polaris "pole star"). It also retains its older name, Cynosura, from a time before it was the pole star, from its Greek name meaning "dog's tail" (as the constellation of Ursa Minor was interpreted as a dog, not a bear, in antiquity).
The North Star can even be seen slightly south of the Equator (because of atmospheric refraction); further south, it cannot be used for navigation. A common method of locating Polaris in the sky is to follow along the line of the so-called "pointer" stars in the bowl of the Big Dipper asterism, specifically, the two stars farthest from its "handle". The arc between the pointer stars and Polaris is nearly five times greater than the arc between the pointer stars.
Due to the precession of the equinoxes (as well as the stars' proper motions), the role of North Star passes from one star to another.
In 3000 BC, the faint star Thuban in the constellation Draco was the North Star. At magnitude 3.67 (fourth magnitude) it is only one-fifth as bright as Polaris, and today it is invisible in light-polluted urban skies.
During the 1st millennium BC, β Ursae Minoris was the bright star closest to the celestial pole, but it was never close enough to be taken as marking the pole, and the Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars.
α Ursae Minoris was described as αει φανης "always visible" by Stobaeus in the 5th century, when it was still removed from the celestial pole by about 8°. It was known as scip-steorra ("ship-star") in 10th-century Anglo-Saxon England, reflecting its use in navigation.
The name stella polaris has been given to α Ursae Minoris since at least the 16th century, even though at that time it was still several degrees away from the celestial pole. Gemma Frisius determined this distance as 3°7' in the year 1547.
The precession of the equinoxes takes about 25,770 years to complete a cycle. Polaris' 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 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.
Gamma Cephei (also known as Alrai, situated 45 light-years away) will become closer to the northern celestial pole than Polaris around 3000 AD. Iota Cephei will become the pole star some time around 5200 AD. First-magnitude Deneb will be within 5° of the North Pole in 10000 AD.
When Polaris becomes the North Star again around 27800 AD, due to its proper motion it then will be farther away from the pole than it is now, while in 23600 BC it was closer to the pole.
Currently, there is no South Star as useful as Polaris. Sigma Octantis is the naked-eye star closest to the south Celestial pole, but at apparent magnitude 5.45 it is barely visible on a clear night, making it unusable for navigational purposes. It is a Yellow giant, 275 light years from Earth. Its angular separation from the pole is about 1° (as of 2000[update]). 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. 
Although the south Celestial pole currently lacks a bright star like Polaris to mark its position, slow changes over time (due to the effects of precession) mean that other stars will become southern pole stars.
The Celestial south pole is moving toward the Southern Cross, which has pointed to the south pole for the last 2,000 years or so. As a consequence, the constellation is no longer visible from subtropical northern latitudes, as it was in the time of the ancient Greeks.
There have been many pole stars throughout the millennia. Around 2000 BC, the star Eta Hydri was the nearest bright star to the Celestial south pole. Around 2800 BC, Achernar was only 8 degrees from the south pole.
In the next 7500 years, the south Celestial pole will pass close to the stars Gamma Chamaeleontis (4200 AD), I Carinae, Omega Carinae (5800 AD), Upsilon Carinae, Iota Carinae (Aspidiske, 8100 AD) and Delta Velorum (9200 AD). From the eightieth to the ninetieth centuries, the south Celestial pole will travel through the False Cross. Around 14000 AD when Vega is only 4 degrees from the North Pole, Canopus is only 8 degrees from the South Pole and is, thus circumpolar on the latitude of Bali (8 deg S).
Pole stars of other planets are defined analogously: they are stars (brighter than 6th magnitude, i.e., visible to the naked eye under ideal conditions) 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. (See Poles of astronomical bodies.)
In Japan, the Pole Star was represented by Myōken Bosatsu (妙見菩薩).
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|Look up Pole Star in Wiktionary, the free dictionary.|