Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a bright magnitude 2 star aligned approximately with its northern axis that serves as a pre-eminent star in celestial navigation, and a much dimmer magnitude 5.5 star on its southern axis, Polaris Australis (Sigma Octantis).
From around 1700BC until just after 300AD, Kochab (Beta Ursae Minoris) and Pherkad (Gamma Ursae Minoris) were twin northern pole stars, though neither was as close to the pole as Polaris is now.
History
A method to find the Pole star Polaris at 5x the distance of the two front stars of the Big DipperThe path of the north celestial pole among the stars due to the effect of precession, with dates shownThe path of the south celestial pole among the stars due to the effect of precession
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 Κυνόσουρα "dog's tail"),[1] was used as indicating the northern direction for the purposes of navigation by the Phoenicians.[2] 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 (transliterated as aeiphanes) meaning "always above the horizon", "ever-shining"[3] 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 Vishnu Purana, it is personified under the name Dhruva ("immovable, fixed").
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°8'.[4] 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.
Precession of the equinoxes
Precession of Earth's rotational axis
In 2022 Polaris' mean declination was 89.35 degrees North;[citation needed] (at epoch J2000 it was 89.26 degrees N). So it 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 within a degree of the latitude of the observer. The celestial pole will be nearest Polaris in 2100.[5][6]
Due to the precession of the equinoxes (as well as the stars' proper motions), the role of North Star has passed from one star to another in the remote past, and will pass in the remote future. In 3000BC, the faint star Thuban in the constellationDraco was the North Star, aligning within 0.1° distance from the celestial pole, the closest of any of the visible pole stars.[7][8] 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.
During the 1st millennium BC, Beta Ursae Minoris (Kochab) 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.[5][9] In the Roman era, the celestial pole was about equally distant between Polaris and Kochab.
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", which translates to 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", which is 1629" (or 0.4526°) from the celestial north pole, on 24 March 2100.[6]
Precession will next point the north celestial pole at stars in the northern constellation Cepheus. The pole will drift to space equidistant between Polaris and Gamma Cephei ("Errai") by 3000AD, with Errai reaching its closest alignment with the northern celestial pole around 4200AD.[10][11]Iota Cephei and Beta Cephei will stand on either side of the northern celestial pole some time around 5200AD, before moving to closer alignment with the brighter star Alpha Cephei ("Alderamin") around 7500AD.[10][12]
Precession will then point the north celestial pole at stars in the northern constellation Cygnus. Like Beta Ursae Minoris during the 1st millennium BC, the bright star closest to the celestial pole in the 10th millennium AD, first-magnitude Deneb, will be a distant 7° from the pole, never close enough to be taken as marking the pole,[7] while third-magnitude Delta Cygni will be a more helpful pole star, at a distance of 3° from celestial north, around 11,250AD.[10] Precession will then point the north celestial pole nearer the constellation Lyra, where the second brightest star in the northern celestial hemisphere, Vega, will be a pole star around 14,500AD, though at a distance of 5° from celestial north.[10]
Precession will eventually point the north celestial pole nearer the stars in the constellation Hercules, pointing towards Tau Herculis around 18,400AD.[13] The celestial pole will then return to the stars in constellation Draco (Thuban, mentioned above) before returning to the current constellation, Ursa Minor. When Polaris becomes the North Star again around 27,800AD, due to its proper motion it then will be farther away from the pole than it is now, while in 23,600 BC it was closer to the pole.[citation needed]
Over the course of Earth's 26,000-year axial precession cycle, a series of bright naked eye stars (an apparent magnitude up to +6; a full moon is −12.9) in the northern hemisphere will hold the transitory title of North Star.[10] While other stars might line up with the north celestial pole during the 26,000 year cycle, they do not necessarily meet the naked eye limit needed to serve as a useful indicator of north to an Earth-based observer, resulting in periods of time during the cycle when there is no clearly defined North Star. There will also be periods during the cycle when bright stars give only an approximate guide to "north", as they may be greater than 5° of angular diameter removed from direct alignment with the north celestial pole.[11]
The 26,000 year cycle of North Stars, starting with the current star, with stars that will be "near-north" indicators when no North Star exists during the cycle, including each star's average brightness and closest alignment to the north celestial pole during the cycle:[5][6][7][8][10][11][12][13]
Series of shots showing the rotation of the Earth's axis relative to the south celestial pole. The Magellanic Clouds, Coalsack Nebula, and Southern Cross (next to the Coalsack) are clearly visible. Near the end of the video, the rise of the Moon illuminates the scene. (Argentina, 2014)
Currently, there is no South Pole Star like Polaris, the so-called North Star. Sigma Octantis is the closest near naked-eye star to the south celestial pole, but at apparent magnitude 5.47 it is barely visible on a clear night, making it less useful for casual navigational or astronomy alignment purposes.[15][16] It is a yellow giant 294 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.[17][18] The celestial south pole is moving toward the Southern Cross, which has pointed to the south pole for the last 2000 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.[19]
Around 200 BC, the star Beta Hydri was the nearest bright star to the celestial south pole.[20] Around 2800 BC, Achernar was only 8 degrees from the south pole.
Circle of southern stars, Chile, 2016
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 (Alsephina, 9200 AD).[21] From the eightieth to the ninetieth centuries, the south celestial pole will travel through the False Cross. Around 14,000AD Canopus will have a declination of –82°, meaning it will rise and set daily for latitudes between 8°S and 8°N, and will not rise to viewers north of this latter 8th parallel north.[22]
Precession and proper motion mean that Sirius will be a future southern pole star: at 88.4° S declination in the year 66,270AD; and 87.7° S declination in the year 93,830AD.[23]
Other planets
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.)
The IAU uses the right-hand rule to define a positive pole for the purpose of determining orientation. Using this convention, Venus is tilted 177° ("upside down").[25]
Due to axial precession, the lunar pole describes a small circle on the celestial sphere every 18.6 years. e.g. Moore, Patrick (1983), The Guinness Book of Astronomy Facts & Feats, p.29, In 1968 the north pole star of the Moon was Omega Draconis; by 1977 it was 36 Draconis. The south pole star is Delta Doradus.
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. Bartholomaeus Anglicus (d. 1272), in the translation of John Trevisa (1397):
by the place of this sterre place and stedes and boundes of the other sterres and of cercles of heven ben knowen: therefore astronomers beholde mooste this sterre. Then this ster is dyscryved of the moste shorte cercle; for he is ferre from the place that we ben in; he hydeth the hugenesse of his quantite for unmevablenes of his place, and he doth cerfifie men moste certenly, that beholde and take hede therof; and therfore he is called stella maris, the sterre of the see, for he ledeth in the see men that saylle and have shyppemannes crafte.[27]
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);[28] it probably arises in the Carolingian era; a late 9th-century manuscript of Jerome's text still has stilla, not stella,[29] 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."[30]
In Mandaean cosmology, the Pole Star is considered to be auspicious and is associated with the World of Light ("heaven"). Mandaeans face north when praying, and temples are also oriented towards the north. On the contrary, the south is associated with the World of Darkness.[31]
Crux is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name Crux is Latin for cross. Even though it is the smallest of all 88 modern constellations, Crux is among the most easily distinguished as its four main stars each have an apparent visual magnitude brighter than +2.8. It has attained a high level of cultural significance in many Southern Hemisphere states and nations.
Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was the southern foundation of the larger constellation of Argo Navis until it was divided into three pieces, the other two being Puppis, and Vela.
Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris and is commonly called the North Star or Pole Star. With an apparent magnitude that fluctuates around 1.98, it is the brightest star in the constellation and is readily visible to the naked eye at night. The position of the star lies less than 1° away from the north celestial pole, making it the current northern pole star. The stable position of the star in the Northern Sky makes it useful for navigation.
Thuban, with Bayer designation Alpha Draconis or α Draconis, is a binary star system in the northern constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star from the 4th to 2nd millennium BC.
Ursa Major, also known as the Great Bear, is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater bear", referring to and contrasting it with nearby Ursa Minor, the lesser bear. In antiquity, it was one of the original 48 constellations listed by Ptolemy in the 2nd century AD, drawing on earlier works by Greek, Egyptian, Babylonian, and Assyrian astronomers. Today it is the third largest of the 88 modern constellations.
Ursa Minor, also known as the Little Bear, is a constellation located in the far northern sky. As with the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, hence the North American name, Little Dipper: seven stars with four in its bowl like its partner the Big Dipper. Ursa Minor was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Ursa Minor has traditionally been important for navigation, particularly by mariners, because of Polaris being the north pole star.
The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at Earth's North Pole and South Pole, respectively. As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per day.
In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particular, axial precession can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years. This is similar to the precession of a spinning top, with the axis tracing out a pair of cones joined at their apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude.
In observational astronomy, culmination is the passage of a celestial object across the observer's local meridian. These events are also known as meridian transits, used in timekeeping and navigation, and measured precisely using a transit telescope.
Leo Minor is a small and faint constellation in the northern celestial hemisphere. Its name is Latin for "the smaller lion", in contrast to Leo, the larger lion. It lies between the larger and more recognizable Ursa Major to the north and Leo to the south. Leo Minor was not regarded as a separate constellation by classical astronomers; it was designated by Johannes Hevelius in 1687.
Kochab, Bayer designation Beta Ursae Minoris, is the brightest star in the bowl of the Little Dipper asterism, and only slightly fainter than Polaris, the northern pole star and brightest star in Ursa Minor. Kochab is 16 degrees from Polaris and has an apparent visual magnitude of 2.08. The distance to this star from the Sun can be deduced from the parallax measurements made during the Hipparcos mission, yielding a value of 130.9 light-years.
Sigma Octantis is a solitary star in the Octans constellation that forms the pole star of the Southern Hemisphere. Its name is also written as σ Octantis, abbreviated as Sigma Oct or σ Oct, and it is officially named Polaris Australis. The star is positioned one degree away from the southern celestial pole of the Southern Hemisphere, lying in nearly opposite direction to the North Star on the celestial sphere.
An asterism is an observed pattern or group of stars in the sky. Asterisms can be any identified pattern or group of stars, and therefore are a more general concept than the 88 formally defined constellations. Constellations are based on asterisms, but unlike asterisms, constellations outline and today completely divide the sky and all its celestial objects into regions around their central asterisms. For example, the asterism known as the Big Dipper comprises the seven brightest stars in the constellation Ursa Major. Another is the asterism of the Southern Cross, within the constellation of Crux.
Alpha Cephei, officially named Alderamin, is a second magnitude star in the constellation of Cepheus near the northern pole. The star is relatively close to Earth at 49 light years (ly) and drifting closer with a heliocentric radial velocity of about −16 km/s.
Gamma Ursae Minoris, also named Pherkad, is a star in the northern constellation of Ursa Minor. Together with Beta Ursae Minoris (Kochab), it forms the end of the dipper pan of the "Little Dipper", which is an asterism forming the tail of the bear. Based upon parallax measurements obtained during the Hipparcos mission, it is approximately 487 light-years from the Sun.
Beta Cephei is a triple star system of the third magnitude in the constellation of Cepheus. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 690 light-years distant from the Sun. It is the prototype of the Beta Cephei variable stars.
The Indestructibles was the name given by ancient Egyptian astronomers to two bright stars which, at that time, could always be seen circling the North Pole. The name is directly related to Egyptian belief in constant North as a portal to heaven for pharaohs, and the stars' close association with eternity and the afterlife. These circumpolar stars are now known as Kochab, in the bowl of Ursa Minor or, the Little Dipper, and Mizar, in Ursa Major, at the middle of the handle of the Big Dipper.
Struve 1694 is a double star in the constellation Camelopardalis.
Tianhuang Emperor is an ancient Chinese constellation, located in the Purple Forbidden enclosure, surrounded by the Gouchen, is one of the symbols of the emperor in astrology . Although the present-day Polaris is located at the tip of the handle of a ladle, around 1100 B.C. the north pole distance of Beta Ursa Minoris was about 6.5 degrees and was the closest Pole Star to the North Pole of the heavens and was called Emperor.
↑ implied by Johannes Kepler (cynosurae septem stellas consideravit quibus cursum navigationis dirigebant Phoenices): "Notae ad Scaligeri Diatribam de Aequinoctiis" in Kepleri Opera Omnia ed. Ch. Frisch, vol. 8.1 (1870) p. 290
1 2 3 Ridpath, Ian, ed. (2004). Norton's Star Atlas. New York: Pearson Education. p.5. ISBN0-13-145164-2. Around 4800 years ago Thuban (α Draconis) lay a mere 0°.1 from the pole. Deneb (α Cygni) will be the brightest star near the pole in about 8000 years' time, at a distance of 7°
1 2 Moore, Patrick (2005). The Observer's Year: 366 Nights in the Universe. p.283.
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