Pole star

Pole star or polar star refers to a star, preferably bright, closely aligned to the axis of rotation of an astronomical object. This is most commonly used for stars close to the north and south celestial poles of the Earth. The name is frequently applied to Polaris in the constellation Ursa Minor, acknowledging its property (extant since the 10th century) 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 naked-eye stars at different times (with an additional slight shift due to the proper motion of the stars).

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):

Polaris was associated with Marian veneration from an early time, Our Lady, Star of the Sea being a title of the Blessed Virgin. This tradition goes back to a misreading of Saint Jerome's translation of Eusebius' Onomasticon, De nominibus hebraicis (written ca. 390). Jerome gave stilla maris "drop of the sea" as a (false) Hebrew etymology of the name Maria. This stilla maris was later misread as stella maris; the misreading is also found in the manuscript tradition of Isidore's Etymologiae (7th century); it probably arises in the Carolingian era; a late 9th-century manuscript of Jerome's text still has stilla, not stella, but Paschasius Radbertus, also writing in the 9th century, makes an explicit reference to the "Star of the Sea" metaphor, saying that Mary is the "Star of the Sea" to be followed on the way to Christ, "lest we capsize amid the storm-tossed waves of the sea."

This page was last edited on 16 May 2018, at 17:27.
Reference: https://en.wikipedia.org/wiki/Pole_star under CC BY-SA license.

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