Pole star

A pole star or polar star is a star, preferably bright, closely aligned to the axis of rotation of an astronomical object. A pole star or polar star is a star, preferably bright, closely aligned to the axis of rotation of an astronomical object. Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a magnitude 2 star aligned approximately with its northern axis, and a pre-eminent star in celestial navigation, and Polaris Australis (Sigma Octantis), a much dimmer star. A couple thousand years ago, Kochab and Pherkad were twin northern pole stars, though neither was as close to the pole as Polaris is now. 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. In the Hindu Puranas, it is 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.' The name stella polaris was coined in the Renaissance, even though at that time it was well recognized that it was several degrees away from the celestial pole; Gemma Frisius in the year 1547 determined this distance as 3°7'.An explicit identification of Mary as stella maris with the North Star (Polaris) becomes evident in the title Cynosura seu Mariana Stella Polaris (i.e. 'Cynosure, or the Marian Polar Star'), a collection of Marian poetry published by Nicolaus Lucensis (Niccolo Barsotti de Lucca) in 1655. As of October 2012, Polaris had the declination +89°19′8″ (at 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 true horizon (after correcting for refraction and other factors) is equal to the latitude of the observer to better than one degree. The celestial pole will be nearest Polaris in 2100 and will thereafter become more distant. Due to the precession of the equinoxes (as well as the stars' proper motions), the role of North Star has passed (and will pass) from one star to another in the remote past (and in the remote future). In 3000 BC, the faint star Thuban in the constellation Draco was the North Star, aligning within 0.1° distance from the celestial pole, the closest of any of the visible pole stars. However, 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.