Apole star is a visiblestar that is approximately aligned with theaxis of rotation of anastronomical body; that is, a star whose apparent position is close to one of thecelestial poles. OnEarth, a pole star would lie directly overhead when viewed from theNorth or theSouth Pole.
Currently, Earth's pole stars arePolaris (Alpha Ursae Minoris), a brightmagnitude 2 star aligned approximately with its northern axis that serves as a pre-eminent star incelestial navigation, and a much dimmer magnitude 5.5 star on its southern axis,Polaris Australis (Sigma Octantis).
From around 1700 BC until just after 300 AD,Kochab (Beta Ursae Minoris) andPherkad (Gamma Ursae Minoris) were twin northern pole stars, though neither was as close to the pole as Polaris is now.
Inclassical 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 ofUrsa Minor, in antiquity known asCynosura (Greek Κυνόσουρα "dog's tail"),[1] was used as indicating the northern direction for the purposes of navigation by thePhoenicians.[2] The ancient name of Ursa Minor, anglicized ascynosure, 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 asaeiphanes) meaning "always above the horizon", "ever-shining"[3] byStobaeus in the 5th century, when it was still removed from the celestial pole by about 8°. It was known asscip-steorra ("ship-star") in 10th-centuryAnglo-Saxon England, reflecting its use in navigation. In the VishnuPurana, it is personified under the nameDhruva ("immovable, fixed").
The namestella 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] In his bookCosmographicus Liber of 1524Petrus Apianus published a diagram of the north polar region of the sky on which he labelled the pole star as both Stella Polaris and Stella Maris.[5]An explicit identification of Mary asstella maris with the North Star (Polaris) becomes evident in the titleCynosura 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.
In 2022 Polaris' meandeclination was 89.35 degrees North;[citation needed] (atepoch 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.[6][7]
Due to theprecession 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 3000 BC, the faint starThuban in theconstellationDraco was the North Star, aligning within 0.1°distance from the celestial pole, the closest of any of the visible pole stars.[8][9] However, at magnitude 3.67 (fourth magnitude) it is only one-fifth as bright as Polaris, and today it is invisible inlight-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 navigatorPytheas in ca. 320 BC described the celestial pole as devoid of stars.[6][10] In theRoman 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 ofprecession andproper motion) will reach a maximumdeclination 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 ofnutation andaberration) will be +89°32'50.62", which is 1629" (or 0.4526°) from the celestial north pole, on 24 March 2100.[7]
Precession will next point the north celestial pole at stars in the northern constellationCepheus. The pole will drift to space equidistant between Polaris andGamma Cephei ("Errai") by 3000 AD, with Errai reaching its closest alignment with the northern celestial pole around 4200 AD.[11][12]Iota Cephei andBeta Cephei will stand on either side of the northern celestial pole some time around 5200 AD, before moving to closer alignment with the brighter starAlpha Cephei ("Alderamin") around 7500 AD.[11][13]
Precession will then point the north celestial pole at stars in the northern constellationCygnus. Like Beta Ursae Minoris during the 1st millennium BC, the bright star closest to the celestial pole in the 10th millennium AD, first-magnitudeDeneb, will be a distant 7° from the pole, never close enough to be taken as marking the pole,[8] while third-magnitudeDelta Cygni will be a more helpful pole star, at a distance of 3° from celestial north, around 11,250 AD.[11] Precession will then point the north celestial pole nearer the constellationLyra, where thesecond brightest star in thenorthern celestial hemisphere,Vega, will be a pole star around 14,500 AD, though at a distance of 5° from celestial north.[11]
Precession will eventually point the north celestial pole nearer the stars in the constellationHercules, pointing towardsTau Herculis around 18,400 AD.[14] 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 27,800 AD, due to itsproper 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-yearaxial precession cycle, a series of brightnaked eye stars (anapparent magnitude up to +6; afull moon is −12.9) in theNorthern Hemisphere will hold the transitory title of North Star.[11] While other stars might line up with the northcelestial 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° ofangular diameter removed from direct alignment with the north celestial pole.[12]
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:[6][7][8][9][11][12][13][14]
| Bayer | Tradi- tional | V | Constel- lation | Align- ment | notes |
|---|---|---|---|---|---|
| Alpha Ursae Minoris | Polaris | 1.98 | Ursa Minor | within 0.5° | the current North Star |
| Gamma Cephei | Errai | 3.21 | Cepheus | within 3° | will become the North Star at about 3,100 AD |
| Iota Cephei | 3.51 | Cepheus | within 5° | shares timing withBeta Cephei | |
| Beta Cephei | Alfirk | 3.51 | Cepheus | within 5° | will become the North Star at about 5,900 AD |
| Alpha Cephei | Alderamin | 2.51 | Cepheus | within 3° | will become the North Star at about 7,500 AD |
| Alpha Cygni | Deneb | 1.25 | Cygnus | within 7° | will become the North Star at about 9,800 AD |
| Delta Cygni | Fawaris | 2.87 | Cygnus | within 3° | will become the North Star at about 11,250 AD |
| Alpha Lyrae | Vega | 0.026 | Lyra | within 5° | used to be the North Star at about 12,000 BC; and will become the North Star at 14,500 AD |
| Iota Herculis | 3.75 | Hercules | within 4° | used to be the North Star at about 9,000 BC; and will become the North Star at 15,000 AD | |
| Tau Herculis | 3.89 | Hercules | within 1° | was the pole star in 7,400 BC, will be again around 18,400 AD | |
| Iota Draconis | Edasich | 3.29 | Draco | within 5° | used to be the North Star at about 4,420 BC |
| Alpha Draconis | Thuban | 3.65 | Draco | within 0.1° | used to be the North Star at about 3,000 BC |
| Kappa Draconis | 3.82 | Draco | within 6° | a near-north star, shares timing withKochab | |
| Beta Ursae Minoris | Kochab | 2.08 | Ursa Minor | within 7° | used to be the North Star at about 1,100 BC |
Currently, there is no South Pole Star likePolaris, the so-calledNorth Star.Sigma Octantis is the closest nearnaked-eye star to the south celestial pole, but atapparent magnitude 5.47 it is barely visible on aclear night, making it less useful for casual navigational or astronomy alignment purposes.[15][16]It is ayellow giant 294light years from Earth. Itsangular separation from the pole is about 1° (as of 2000[update]). TheSouthern Cross constellation functions as an approximate southern pole constellation, by pointing to where a southern pole star would be.
At theequator, 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 theancient Greeks.[19]
Around 200 BC, the starBeta Hydri was the nearest bright star to the celestial south pole.[20] 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 starsGamma Chamaeleontis (4200 AD),I Carinae,Omega Carinae (5800 AD),Upsilon Carinae,Iota Carinae (Aspidiske, 8100 AD) andDelta Velorum (Alsephina, 9200 AD).[21] From the eightieth to the ninetieth centuries, the south celestial pole will travel through theFalse Cross. Around 14,000 ADCanopus 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 latter8th parallel north.[22]
Precession and proper motion mean thatSirius will be a future southern pole star: at 88.4° S declination in the year 66,270 AD; and 87.7° S declination in the year 93,830 AD.[23]
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. (SeePoles of astronomical bodies.)
| Planet | North star | South star | notes |
|---|---|---|---|
| Mercury[24] | Omicron Draconis | Alpha Pictoris | |
| Venus | Eta1 Doradus | 42 Draconis | The IAU uses theright-hand rule to define apositive pole for the purpose of determining orientation. Using this convention, Venus is tilted 177° ("upside down").[25] |
| Moon | Omicron Draconis | Delta Doradus | Due toaxial 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. |
| Mars | The top two stars in theNorthern Cross,Gamma Cygni andDeneb, point to the pole.[26] | Kappa Velorum is a couple of degrees away. | |
| Jupiter | a little over two degrees away fromZeta Draconis | about two degrees north ofDelta Doradus | |
| Saturn | in the far northern region ofCepheus, about six degrees from Polaris | Delta Octantis | |
| Uranus | Eta Ophiuchi | 15 Orionis | |
| Neptune | midway betweenGamma Cygni andDelta Cygni | Gamma Velorum |
In the medieval period, Polaris was also known asstella maris ("star of the sea", from its use for navigation at sea), as in e.g.Bartholomaeus Anglicus (d. 1272), in the translation ofJohn 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 calledstella maris, the sterre of the see, for he ledeth in the see men that saylle and have shyppemannes crafte.[27]
Polaris was associated withMarian 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 ofSaint Jerome's translation ofEusebius'Onomasticon,De nominibus hebraicis (written ca. 390). Jerome gavestilla maris "drop of the sea" as a (false) Hebrew etymology of thenameMaria. Thisstilla maris was later misread asstella maris; the misreading is also found in the manuscript tradition ofIsidore'sEtymologiae (7th century);[28] it probably arises in theCarolingian era; a late 9th-century manuscript of Jerome's text still hasstilla, notstella,[29] butPaschasius 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]
InMandaean cosmology, the Pole Star is considered to be auspicious and is associated with theWorld of Light ("heaven").Mandaeans face north when praying, andtemples are also oriented towards the north. On the contrary, the south is associated with theWorld of Darkness.[31]
InHinduism, the Pole Star is revered and referred to as Dhruva (Sanskrit: ध्रुव, IAST: Dhruva, lit. "unshakeable, immovable, fixed or eternal").Dhruva is considered an ascetic devotee ofVishnu mentioned in the Vishnu Purana and the Bhagavata Purana who ascended to become the Polestar.TheSanskrit term dhruva nakshatra (ध्रुव नक्षत्र, "polar star") has been used for Pole Star in theMahabharata, personified as son ofUttānapāda and grandson ofManu.
...in the early 16th century ... Polaris was still around three and a half degrees from the celestial pole ...will reach its closest to the north celestial pole around AD 2100, when the separation will be less than half a degree
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°
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