Pole star: Difference between revisions - Wikipedia

Tag: Reverted

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[[File:Earth precession.svg|thumb|left|Precession of Earth's rotational axis]]

In 2022 Polaris' mean [[declination]] was 89.35 degrees North;{{citation needed|date=May 2022}} (at [[Epoch (astronomy)#Julian years and J2000|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.<ref name=StarTales>{{cite book |chapter-url=http://www.ianridpath.com/startales/ursaminor.htm#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 CE, 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 BCBCE, 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 BCBCE, [[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 BCBCE 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 ADCE, with Errai reaching its closest alignment with the northern celestial pole around 4200 ADCE.<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 ADCE, before moving to closer alignment with the brighter star [[Alpha Cephei]] ("Alderamin") around 7500 ADCE.<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 BCBCE, the bright star closest to the celestial pole in the 10th millennium ADCE, 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 ADCE.<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 ADCE, 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 ADCE.<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 ADCE, due to its [[proper motion]] it then will be farther away from the pole than it is now, while in 23,600 BCBCE 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/>

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! Notes

|-

| [[Alpha Draconis]] || Thuban || 3.65 || [[Draco (constellation)|Draco]] || within 0.1° || used to be the North Star at about 3,000 BCBCE

|-

| [[Alpha Ursae Minoris]] || Polaris || 1.98 || [[Ursa Minor]] || within 0.5° || the current North Star

|-

| [[Tau Herculis]] || || 3.89 || [[Hercules (constellation)|Hercules]] || within 1° || was the pole star in 7,400 BCBCE, will be again around 18,400 ADCE

|-

| [[Gamma Cephei]] || Errai || 3.21 || [[Cepheus (constellation)|Cepheus]] || within 3° || will become the North Star at about 3,100 ADCE

|-

| [[Alpha Cephei]] || Alderamin || 2.51 || [[Cepheus (constellation)|Cepheus]] || within 3° || will become the North Star at about 7,500 ADCE

|-

| [[Delta Cygni]] || Fawaris || 2.87 || [[Cygnus (constellation)|Cygnus]] || within 3° || will become the North Star at about 11,250 ADCE

|-

| [[Iota Herculis]] || || 3.75 || [[Hercules (constellation)|Hercules]] || within 4° || used to be the North Star at about 9,000 BCBCE; and will become the North Star at 15,000 ADCE

|-

| [[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 Lyrae]] || Vega || 0.026 || [[Lyra]] || within 5° || used to be the North Star at about 12,000 BCBCE; and will become the North Star at 14,500 ADCE

|-

| [[Iota Draconis]] || Edasich || 3.29 || [[Draco (constellation)|Draco]] || within 5° || used to be the North Star at about 4,420 BCBCE

|-

| [[Kappa Draconis]] || || 3.82 || [[Draco (constellation)|Draco]] || within 6° || a near-north star, shares timing with [[Beta Ursae Minoris|Kochab]]

|-

| [[Alpha Cygni]] || Deneb || 1.25 || [[Cygnus (constellation)|Cygnus]] || within 7° || will become the North Star at about 9,800 ADCE

|-

| [[Beta Ursae Minoris]] || Kochab || 2.08 || [[Ursa Minor]] || within 7° || used to be the North Star at about 1,100 BCBCE

|}