Pole star: Difference between revisions - Wikipedia
Article Images
Line 7:
[[File:Star Trail above Beccles - geograph.org.uk - 1855505.jpg|thumb|upright|[[Northern Hemisphere]] [[circumpolar stars]] around [[Polaris]], with a [[long-exposure photography|long-exposure]] producing a [[star trail]] photo]]
A '''pole star''' or '''polar star''' is a visible [[star]], preferablythat bright,is nearlyapproximately aligned with the [[axis of arotation]] of rotatingan [[astronomical object|astronomical body]]; forthat is, a star whose apparent position is close to one of the [[celestial pole]]s. On [[Earth]], thesea 2pole pointsstar arewould calledlie directly overhead when viewed from the [[Celestial_poleNorth Pole|celestialNorth]] or the [[South polesPole]].
Currently, Earth's pole stars are [[Polaris]] (Alpha Ursae Minoris), a bright [[apparent magnitude|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 1700 BC until just after 300 AD, [[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==
[[File:A method to find Polaris at 5x the distance of Merak and Dubhe of Ursa Major.jpg|thumb|A method to find the Pole star Polaris at 5x the distance of the two front stars of the Big Dipper.]]
[[File:Precession N.gif|thumb|upright=1.2|The path of the north celestial pole among the stars due to the effect of precession, with dates shown]]
[[File:Precession S.gif|thumb|upright=1.2|The 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 ''[[:wikt:Cynosura|Cynosura]]'' (Greek Κυνόσουρα "dog's tail"),<ref>{{LSJ|kuno/soura|κυνόσουρα|ref}}.</ref> was used as indicating the northern direction for the purposes of navigation by the [[Phoenicians]].<ref>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) [https://books.google.com/books?id=BRM2AQAAMAAJ&pg=PT290 p. 290]</ref> The ancient name of Ursa Minor, anglicized as ''[[:wikt:cynosure|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"<ref>{{LSJ|a)eifanh/s|ἀειφανής|shortref}}.</ref> 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").
Line 30:
The celestial pole will be nearest Polaris in 2100.<ref name=StarTales>{{cite book |chapter-url=http://www.ianridpath.com/startales/ursaminor.html#polaris | author=Ridpath, Ian | author-link=Ian Ridpath | date=1988 | title=Star Tales | chapter=Chapter Three: The celestial eighty-eight – Ursa Minor | publisher=[[The Lutterworth Press]] | location=[[Cambridge]] | isbn=978-0-7188-2695-6 | quote=...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}}</ref><ref name=Meeus/>
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 (constellation)|Draco]] was the North Star, aligning within 0.1° [[angular diameter|distance]] from the celestial pole, the closest of any of the visible pole stars.<ref name="Nor">{{cite book | editor=Ridpath, Ian | editor-link=Ian Ridpath | date=2004 | title=Norton's Star Atlas | page=[https://archive.org/details/nortonsstaratlas00ianr/page/5 5] | publisher=Pearson Education | location=New York | isbn=0-13-145164-2 | quote=Around 4800 years ago Thuban ({{GreekFont|α}} Draconis) lay a mere 0°.1 from the pole. Deneb ({{GreekFont|α}} Cygni) will be the brightest star near the pole in about 8000 years' time, at a distance of 7° | url-access=registration | url=https://archive.org/details/nortonsstaratlas00ianr/page/5 }}</ref><ref name=MOORE>{{Cite book| title=The Observer's Year: 366 Nights in the Universe | author=Moore, Patrick | page=283 | year=2005}}</ref> However, at magnitude 3.67 (fourth magnitude) it is only one-fifth as bright as Polaris, and today it is invisible in [[Light pollution|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.<ref name=StarTales/><ref name=KalerKochab>{{citation |title=KOCHAB (Beta Ursae Minoris) |work=Stars |publisher=[[University of Illinois]] | author=Kaler, James B. | author-link=James B. Kaler |url=http://stars.astro.illinois.edu/sow/kochab.html |access-date=2018-04-28}}</ref> In the [[Roman Empire|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 [[astronomical nutation|nutation]] and [[aberration of light|aberration]]) will be +89°32'50.62", which is 1629" (or 0.4526°) from the celestial north pole, on 24 March 2100.<ref name=Meeus>Jean Meeus, Mathematical Astronomy Morsels Ch. 50; Willmann-Bell 1997</ref>
Precession will next point the north celestial pole at stars in the northern constellation [[Cepheus (constellation)|Cepheus]]. The pole will drift to space equidistant between Polaris and [[Gamma Cephei]] ("Errai") by 3000 AD, with Errai reaching its closest alignment with the northern celestial pole around 4200 AD.<ref name=Monthly1871/><ref name=EarthSky>{{cite web |url=http://earthsky.org/brightest-stars/star-errai-future-north-star |title=Gamma Cephei: A future Pole Star |first1=Bruce |last1=McClure |author2=Deborah, Byrd |author2-link=Deborah Byrd |work=[[Earth & Sky|EarthSky]] |date=2017-09-29 |access-date=2018-04-25}}</ref> [[Iota Cephei]] and [[Beta Cephei]] will stand on either side of the northern celestial pole some time around 5200 AD, before moving to closer alignment with the brighter star [[Alpha Cephei]] ("Alderamin") around 7500 AD.<ref name=Monthly1871/><ref name=KalerAlderamin>{{citation |title=ALDERAMIN (Alpha Cephei) |work=Stars |publisher=[[University of Illinois]] | author=Kaler, James B. | author-link=James B. Kaler |url=http://stars.astro.illinois.edu/sow/alderamin.html |access-date=2018-04-28}}</ref>
Precession will then point the north celestial pole at stars in the northern constellation [[Cygnus (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,<ref name="Nor"/> while third-magnitude [[Delta Cygni]] will be a more helpful pole star, at a distance of 3° from celestial north, around 11,250 AD.<ref name=Monthly1871/> Precession will then point the north celestial pole nearer the constellation [[Lyra]], where the [[List of brightest stars|second brightest star]] in the [[northern celestial hemisphere]], [[Vega]], will be a pole star around 14,500 AD, though at a distance of 5° from celestial north.<ref name=Monthly1871/>
Precession will eventually point the north celestial pole nearer the stars in the constellation [[Hercules (constellation)|Hercules]], pointing towards [[Tau Herculis]] around 18,400 AD.<ref name=KalerTauHer>{{citation | title=TAU HER (Tau Herculis) | publisher=[[University of Illinois]] | author=Kaler, James B. | author-link=James B. Kaler | work=Stars | url=http://stars.astro.illinois.edu/sow/tauher.html | access-date=2018-04-27}}</ref>
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 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|date=February 2012}}
Over the course of Earth's 26,000-year [[axial precession]] cycle, a series of bright [[Naked eye#Naked eye in astronomy|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.<ref name=Monthly1871>{{citation | title=Our Monthly | year=1871 | volume=4 | page=53 | publisher=Presbyterian Magazine Company | postscript=. | url=https://books.google.com/books?id=KdEQAAAAIAAJ&pg=PA53 }}</ref> 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.<ref name=EarthSky/>
Line 48:
{| class="wikitable sortable"
! [[Bayer designation|Bayer]]
! Tradi-<br>tional
! Traditional
! [[Apparent magnitude|V]]
! [[Constellation|Constel-<br>lation]]
! [[Celestial pole|AlignmentAlign-<br>ment]]
! notes
|-
| [[Alpha Ursae Minoris]] || Polaris || 1.98 || [[Ursa Minor]] || within 0.5° || the current North Star
|-
| [[Gamma Cephei]] || Errai || 3.21 || [[Cepheus (constellation)|Cepheus]] || within 3° || will become the North Star at about 3,100 ADCE<ref>{{Cite web |title=Gamma Cephei, aka Errai, a future North Star |url=https://earthsky.org/brightest-stars/gamma-cephei-errai-future-north-star/#:~:text=And%20interestingly%2C%20by%204%2C000%20CE,position%20as%20our%20North%20Star.}}</ref>
|-
| [[Iota Cephei]] || || 3.51 || [[Cepheus (constellation)|Cepheus]] || within 5° || shares timing with [[Beta Cephei]]
|-
| [[Beta Cephei]] || Alfirk || 3.51 || [[Cepheus (constellation)|Cepheus]] || within 5° || will become the North Star at about 5,900 ADCE
|-
| [[Alpha Cephei]] || Alderamin || 2.51 || [[Cepheus (constellation)|Cepheus]] || within 3° || will become the North Star at about 7,500 ADCE
|-
| [[Alpha Cygni]] || Deneb || 1.25 || [[Cygnus (constellation)|Cygnus]] || within 7° || will become the North Star at about 9,800 ADCE
|-
| [[Delta Cygni]] || Fawaris || 2.87 || [[Cygnus (constellation)|Cygnus]] || within 3° || will become the North Star at about 11,250 ADCE
|-
| [[Alpha Lyrae]] || Vega || 0.026 || [[Lyra]] || within 5° || used to be the North Star at about 12,000 BC; BCE<br>and will become the North Star at 14,500 ADCE
|-
| [[Iota Herculis]] || || 3.75 || [[Hercules (constellation)|Hercules]] || within 4° || used to be the North Star at about 9,000 BC; BCE<br>and will become the North Star at 15,000 ADCE
|-
| [[Tau Herculis]] || || 3.89 || [[Hercules (constellation)|Hercules]] || within 1° || was the pole star in 7,400 BC, BCE<br>will be again around 18,400 ADCE
|-
| [[Iota Draconis]] || Edasich || 3.29 || [[Draco (constellation)|Draco]] || within 5° || used to be the North Star at about 4,420 BCBCE
|-
| [[Alpha Draconis]] || Thuban || 3.65 || [[Draco (constellation)|Draco]] || within 0.1° || used to be the North Star at about 3,000 BCBCE
|-
| [[Kappa Draconis]] || || 3.82 || [[Draco (constellation)|Draco]] || within 6° || a near-north star, shares timing with [[Beta Ursae Minoris|Kochab]]
|-
| [[Beta Ursae Minoris]] || Kochab || 2.08 || [[Ursa Minor]] || within 7° || used to be the North Star at about 1,100 BCBCE
|}
==Southern pole star (South Star)==
[[File:South Celestial Pole.ogv|right|thumb|upright=1.2|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 moonMoon 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 [[Bortle Scale|clear night]], making it less useful for casual navigational or astronomy alignment purposes<!-- and although sometimes referred to as [[Polaris Australis]] is not technically a pole star-->.<ref>{{cite web
|url =http://jumk.de/astronomie/special-stars/sigma-octantis.shtml
Line 112:
Around 200 BC, the star [[Beta Hydri]] was the nearest bright star to the celestial south pole.<ref>{{Cite web|url=http://stars.astro.illinois.edu/Sow/betahyi.html|title=Beta Hydri}}</ref> Around 2800 BC, [[Achernar]] was only 8 degrees from the south pole.
[[File:All In A Spin Star trail.jpg|thumb|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), [[HR 4102|I Carinae]], [[Omega Carinae]] (5800 AD), [[Upsilon Carinae]], [[Iota Carinae]] (Aspidiske, 8100 AD) and [[Delta Velorum]] (Alsephina, 9200 AD).<ref>{{cite web|url=http://moonkmft.co.uk/Precession.html|title=Precession|website=moonkmft.co.uk|access-date=24 September 2018}}</ref> From the eightieth to the ninetieth centuries, the south celestial pole will travel through the [[False Cross]]. Around 14,000 AD [[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]].<ref>{{cite web
|url =http://moonkmft.co.uk/Precession.html
| title =Precession
Line 120:
|access-date=2018-09-24}}</ref>
Precession and proper motion mean that [[Sirius]] 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.<ref>{{cite web
|url =http://earthsky.org/?p=226733
| title =Sirius, future South Pole Star
Line 130:
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]].)
{| class="wikitable"
* [[Alpha Pictoris]] is the south pole star of [[Mercury (planet)|Mercury]] while [[Omicron Draconis]] is its north star.<ref>2004. [[Starry Night (planetarium software)|Starry Night Pro]], Version 5.8.4. [[Imaginova]]. {{ISBN|978-0-07-333666-4}}. www.starrynight.com</ref>▼
! Planet !! North star !! South star !! notes
* [[42 Draconis]] is the closest star to the north pole of [[Venus]]. [[Eta¹ Doradus]] is the closest to the south pole. (Note: The IAU uses the [[Axis–angle representation|right-hand rule]] to define a ''positive pole'' for the purpose of determining orientation. Using this convention, Venus is tilted 177° ("upside down").)<ref>{{cite journal▼
|-
▲*| [[Alpha Pictoris]] is the south pole star of [[Mercury (planet)|Mercury]] while [[Omicron Draconis]] is its north star.<ref>2004. [[Starry Night (planetarium software)|Starry Night Pro]], Version 5.8.4. [[Imaginova]]. {{ISBN|978-0-07-333666-4}}. www.starrynight.com</ref>|| [[Omicron Draconis]] || [[Alpha Pictoris]]
|-
| Venus || [[42 Draconis|Fafnir]] || [[Eta1 Doradus|Eta<sup>1</sup> Doradus]]
▲* [[42 Draconis]] is the closest star to the north pole of [[Venus]]. [[Eta¹ Doradus]] is the closest to the south pole. (Note:| The IAU uses the [[Axis–angle representation|right-hand rule]] to define a ''positive pole'' for the purpose of determining orientation. Using this convention, Venus is tilted 177° ("upside down").)<ref>{{cite journal
| doi = 10.1007/s10569-010-9320-4
| title = Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009
Line 183 ⟶ 188:
| url-status = dead
}}</ref>
|-
*| TheMoon lunar|| south[[Omicron poleDraconis]] star is|| [[Delta Doradus]], and|| the north pole star{{NoteTag|Due to [[axial precession]], the lunar pole describes a small circle on the celestial sphere every 18.6 years. e.g. {{citation |title = The Guinness Book of Astronomy Facts & Feats |page = 29 |author-link = Patrick Moore |first = Patrick |last = Moore |year = 1983 |quote = 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.}}}} [[Omicron Draconis]].
* [[Kappa Velorum]] is only a couple of degrees from the south celestial pole of [[Mars]]. The top two stars in the [[Cygnus (constellation)|Northern Cross]], [[Gamma Cygni|Sadr]] and [[Deneb]], point to the north celestial pole of Mars.<ref name="Barlow">{{cite book |last=Barlow |first=N. G. |url=https://archive.org/details/marsintroduction00barl_258 |title=Mars: An introduction to its interior, surface and atmosphere |date=2008 |publisher=[[Cambridge University Press]] |isbn=978-0-521-85226-5 |page=[https://archive.org/details/marsintroduction00barl_258/page/n30 21] |url-access=limited}}</ref>▼
|-
* The north pole of [[Jupiter]] is a little over two degrees away from [[Zeta Draconis]], while its south pole is about two degrees north of [[Delta Doradus]].▼
▲*| [[KappaMars Velorum]] is only a couple of degrees from the south celestial pole of [[Mars]].|| The top two stars in the [[Cygnus (constellation)|Northern Cross]], [[Gamma Cygni|Sadr]] and [[Deneb]], point to the north celestial pole of Mars.<ref name="Barlow">{{cite book |last=Barlow |first=N. G. |url=https://archive.org/details/marsintroduction00barl_258 |title=Mars: An introduction to its interior, surface and atmosphere |date=2008 |publisher=[[Cambridge University Press]] |isbn=978-0-521-85226-5 |page=[https://archive.org/details/marsintroduction00barl_258/page/n30 21] |url-access=limited}}</ref>
* [[Delta Octantis]] is the south pole star of [[Saturn]]. Its north pole is in the far northern region of [[Cepheus (constellation)|Cepheus]], about six degrees from Polaris.▼
| [[Kappa Velorum|Markeb]] is a couple of degrees away.
* [[Eta Ophiuchi]] is the north pole star of [[Uranus]], and [[15 Orionis]] is its south pole star.
|-
* The north pole of [[Neptune]] points to a spot midway between [[Gamma Cygni|Gamma]] and [[Delta Cygni]]. Its south pole star is [[Gamma Velorum]].▼
▲*| The north pole of [[Jupiter]] is|| a little over two degrees away from [[Zeta Draconis|Aldhibah]], while its south pole is|| about two degrees north of [[Delta Doradus]].
|-
▲*| [[Delta Octantis]] is the south pole star of [[Saturn]]. Its north pole is|| in the far northern region of [[Cepheus (constellation)|Cepheus]], about six degrees from Polaris. || [[Delta Octantis]]
|-
| Uranus || [[Eta Ophiuchi|Sabik]] || [[15 Orionis]]
|-
▲*| The north pole of [[Neptune]] points to a spot|| midway between [[Gamma Cygni|GammaSadr]] and [[Delta Cygni|Fawaris]]. Its south pole star is|| [[Gamma Velorum|Regor]].
|}
==In religion and mythology==
[[File:Utsjoki.vaakuna.svg|thumb|upright=0.7|The [[North Star]] pictured in the coat of arms of [[Utsjoki]]]]
In the medieval period, Polaris was also known as [[Our Lady, Star of the Sea|''stella maris'']] ("star of the sea", from its use for navigation at sea), as in e.g. [[BartholomeusBartholomaeus Anglicus]] (d. 1272), in the translation of [[John Trevisa]] (1397):
{{blockquote|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.<ref>cited after J. O. Halliwell, (ed.), ''The Works of William Shakespeare'' vol. 5 (1856), [https://books.google.com/books?id=7NVfAAAAcAAJ&pg=PA40 p. 40].]</ref>}}
Line 203 ⟶ 216:
==See also==
*[[Astronomy on Mars#Celestial poles and ecliptic|Astronomy on Mars § Celestial poles and ecliptic]]
*[[Astrotheology]]
*[[Celestial equator]]
*[[Direction determination]]
* [[Empirical evidence for the spherical shape of Earth#Observation of certain, fixed stars from different locations|Empirical evidence for the spherical shape of Earth § Observation of certain, fixed stars from different locations]]
*[[Guide star]]
*[[Lists of stars]]
*[[Worship of heavenly bodies]]
==Notes==
Line 222 ⟶ 236:
{{Star}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{DEFAULTSORT:Pole Star}}