Pole star

Pole star or polar star is a name of Polaris in the constellation Ursa Minor, after its property of being the naked-eye star closest to the Earth's north celestial pole. The name Polaris, introduced in the 18th century, is shortened from New Latin stella polaris, meaning "pole star". Polaris is also known as Lodestar, Guiding Star, or North Star from its property of remaining in a fixed position throughout the course of the night and its use in celestial navigation. It is a dependable, though inexact, indicator of the direction toward the geographic north pole; it is virtually fixed, and its angle of elevation can also be used to determine latitude. The south celestial pole lacks a bright star like Polaris to mark its position. At present, the naked-eye star nearest to the celestial south pole is the faint Sigma Octantis, sometimes called the South Star.

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 Earth's rotational axis, which causes its orientation to change over time. Precession causes the celestial poles to trace out circles on the celestial sphere approximately once every 26,000 years, passing close to different stars at different times (with an additional slight shift due to the proper motion of the stars).

In a more general sense, a pole star may be any fixed star close to either celestial pole of any given planetary body.

In classical antiquity, Beta Ursae Minoris (Kochab) was closer to the celestial north pole than Alpha Ursae Minoris. While there was no naked-eye star close to the pole, the midpoint between Alpha and Beta Ursae Minoris was reasonably close to the pole, and it appears that the entire constellation of Ursa Minor, in antiquity known as Cynosura (Greek Κυνοσούρα "dog's tail") was used as indicating the northern direction for the purposes of navigation by the Phoenicians. The ancient name of Ursa Minor, anglicized as cynosure, has since itself become a term for "guiding principle" after the constellation's use in navigation.

Alpha Ursae Minoris (Polaris) was described as ἀειφανής "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. At around the same time, in the Hindu Puranas, it became personified under the name Dhruva ("immovable, fixed").

In the medieval period, Polaris was also known as stella maris "star of the sea" (from its use for navigation at sea), as in e.g. Bartholomeus Anglicus (d. 1272), in the translation of John Trevisa (1397):

This page was last edited on 7 February 2018, at 09:15.
Reference: https://en.wikipedia.org/wiki/Pole_star under CC BY-SA license.

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