Editing Polaris (section)

== Observation ==

===Variability===
[[File:AlphaUMiLightCurve.png|thumb|left|A [[light curve]] for Polaris, plotted from ''[[Transiting Exoplanet Survey Satellite|TESS]]'' data<ref name=MAST>{{cite web |title=MAST: Barbara A. Mikulski Archive for Space Telescopes |url=https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html |publisher=Space Telescope Science Institute |access-date=8 December 2021}}</ref>]]
Polaris Aa, the supergiant primary component, is a low-amplitude [[Population I]] [[classical Cepheid variable]], although it was once thought to be a [[type II Cepheid]] due to its high [[galactic latitude]]. Cepheids constitute an important [[standard candle]] for determining distance, so Polaris, as the closest such star,<ref name=Anderson2019/> is heavily studied. The [[Variable star|variability]] of Polaris had been suspected since 1852; this variation was confirmed by [[Ejnar Hertzsprung]] in 1911.<ref>{{cite journal
 | last1=Hertzsprung | first1=Ejnar
 | title=Nachweis der Veränderlichkeit von α Ursae Minoris
 | journal=Astronomische Nachrichten | volume=189 | page=89
 | date=August 1911 | language=de
 | doi=10.1002/asna.19111890602 | bibcode=1911AN....189...89H
 | issue=6 | url=https://zenodo.org/record/1424878
 }}</ref>

The range of brightness of Polaris is given as 1.86–2.13,<ref name=gcvs/> but the amplitude has changed since discovery. Prior to 1963, the amplitude was over 0.1 magnitude and was very gradually decreasing. After 1966, it very rapidly decreased until it was less than 0.05 magnitude; since then, it has erratically varied near that range. It has been reported that the amplitude is now increasing again, a reversal not seen in any other Cepheid.<ref name=lee/>

[[File:Integrated Flux Nebula Surrounding Polaris - Kush Chandaria.jpg|thumb|left|Polaris and its surrounding [[Integrated Flux Nebula|integrated flux nebula]]]]
The period, roughly 4 days, has also changed over time. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965. This was originally thought to be due to secular redward (a long term change in [[redshift]] that causes light to stretch into longer wavelengths, causing it to appear red) evolution across the Cepheid [[instability strip]], but it may be due to interference between the primary and the first-[[overtone]] pulsation modes.<ref name=evans>{{Cite journal | last1 = Evans | first1 = N. R. | last2 = Sasselov | first2 = D. D. | last3 = Short | first3 = C. I. | doi = 10.1086/338583 | title = Polaris: Amplitude, Period Change, and Companions | journal = The Astrophysical Journal | volume = 567 | issue = 2 | pages = 1121 | year = 2002 |bibcode = 2002ApJ...567.1121E | doi-access = free }}</ref><ref name=turner>{{Cite journal | last1 = Turner | first1 = D. G. | last2 = Savoy | first2 = J. | last3 = Derrah | first3 = J. | last4 = Abdel-Sabour Abdel-Latif | first4 = M. | last5 = Berdnikov | first5 = L. N. | title = The Period Changes of Polaris | doi = 10.1086/427838 | journal = Publications of the Astronomical Society of the Pacific | volume = 117 | issue = 828 | pages = 207 | year = 2005 |bibcode = 2005PASP..117..207T | doi-access = free }}</ref><ref name=neilson>{{Cite journal | last1 = Neilson | first1 = H. R. | last2 = Engle | first2 = S. G. | last3 = Guinan | first3 = E. | last4 = Langer | first4 = N. | last5 = Wasatonic | first5 = R. P. | last6 = Williams | first6 = D. B. | doi = 10.1088/2041-8205/745/2/L32 | title = The Period Change of the Cepheid Polaris Suggests Enhanced Mass Loss | journal = The Astrophysical Journal | volume = 745 | issue = 2 | pages = L32 | year = 2012 |arxiv = 1201.0761 |bibcode = 2012ApJ...745L..32N | s2cid = 118625176 }}</ref> Authors disagree on whether Polaris is a fundamental or first-overtone pulsator and on whether it is crossing the instability strip for the first time or not.<ref name=fadeyev/><ref name=neilson/><ref name=engle>{{cite journal|doi=10.3847/2515-5172/aad2d0|title=Toward Ending the Polaris Parallax Debate: A Precise Distance to Our Nearest Cepheid from Gaia DR2|journal=Research Notes of the AAS|volume=2|issue=3|pages=126|year=2018|last1=Engle|first1=Scott G|last2=Guinan|first2=Edward F|last3=Harmanec|first3=Petr|bibcode=2018RNAAS...2..126E|s2cid=126329676 |doi-access=free }}</ref>

The temperature of Polaris varies by only a small amount during its pulsations, but the amount of this variation is variable and unpredictable. The erratic changes of temperature and the amplitude of temperature changes during each cycle, from less than 50&nbsp;[[kelvin|K]] to at least 170&nbsp;K, may be related to the orbit with Polaris Ab.<ref name=usenko2005/>

Research reported in ''[[Science (journal)|Science]]'' suggests that Polaris is 2.5 times brighter today than when [[Ptolemy]] observed it, changing from third to second magnitude.<ref>{{cite journal|doi=10.1126/science.304.5678.1740b|pmid=15205508|year=2004|last1=Irion|first1=R|title=American Astronomical Society meeting. As inconstant as the Northern Star|journal=Science|volume=304|issue=5678|pages=1740–1|s2cid=129246155}}</ref> Astronomer [[Edward Guinan]] considers this to be a remarkable change and is on record as saying that "if they are real, these changes are 100 times larger than [those] predicted by current theories of [[Stellar Evolution|stellar evolution]]".

In 2024, researchers led by Nancy Evans at the [[Harvard–Smithsonian Center for Astrophysics|Harvard & Smithsonian]], have studied with more accuracy the Polaris' smaller companion orbit using the [[CHARA Array]]. During this observation campaign they have succeeded in shooting Polaris features on its surface; large bright places and dark ones have appeared in close-up images, changing over time. Further, Polaris diameter size has been re-measured to {{solar radius|46}}, using the ''Gaia'' distance of {{val|446|1}} light-years, and its mass was determined at {{solar mass|5.13}}.<ref name=evans2024>{{Cite journal |last1=Evans |first1=Nancy Remage |last2=Schaefer |first2=Gail H. |last3=Gallenne |first3=Alexandre |last4=Torres |first4=Guillermo |last5=Horch |first5=Elliott P. |last6=Anderson |first6=Richard I. |last7=Monnier |first7=John D. |last8=Roettenbacher |first8=Rachael M. |last9=Baron |first9=Fabien |last10=Anugu |first10=Narsireddy |last11=Davidson |first11=James W. |last12=Kervella |first12=Pierre |last13=Bras |first13=Garance |last14=Proffitt |first14=Charles |last15=Mérand |first15=Antoine |date=2024-08-01 |title=The Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array |bibcode=2024ApJ...971..190E |journal=The Astrophysical Journal |volume=971 |issue=2 |pages=190 |doi=10.3847/1538-4357/ad5e7a |doi-access=free |arxiv=2407.09641 |issn=0004-637X}}</ref>

===Role as pole star===
{{main|Pole star}}
[[File:Polaris-clock face.jpg|thumb|left|Polaris azimuths vis clock face analogy.<ref name="kaizad_co_uk">{{cite web | url=http://www.kaizad.co.uk/data/Nav/Polaris.xls | title=A visual method to correct a ship's compass using Polaris using Ursa Major as a point of reference | access-date=2016-08-07 | archive-date=2010-08-27 | archive-url=https://web.archive.org/web/20100827042213/http://www.kaizad.co.uk/data/Nav/Polaris.xls | url-status=dead }}</ref>]]
[[File:Star Trail above Beccles - geograph.org.uk - 1855505.jpg|thumb|A typical Northern Hemisphere [[star trail]] with Polaris in the center.]]
[[File:Dipper polaris cass.png|thumb|Polaris lying halfway between the [[Asterism (astronomy)|asterisms]] [[Cassiopeia (constellation)|Cassiopeia]] and the [[Big Dipper]].]]
Because Polaris lies nearly in a direct line with the [[Earth's rotation]]al axis above the [[North Pole]], it stands almost motionless in the sky, and all the stars of the northern sky appear to rotate around it. It thus provides a nearly fixed point from which to draw measurements for [[celestial navigation]] and for [[astrometry]]. The elevation of the star above the horizon gives the approximate [[latitude]] of the observer.<ref name=Kaler/>

In 2018 Polaris was 0.66° (39.6 arcminutes) away from the pole of rotation (1.4 times the [[Moon]] disc) and so revolves around the pole in a small circle 1.3° in diameter. It will be closest to the pole (about 0.45 degree, or 27 arcminutes) soon after the year 2100.<ref>{{cite journal |bibcode=1990JBAA..100..212M |title=Polaris and the North Pole |last1=Meeus |first1=J. |journal=Journal of the British Astronomical Association |year=1990 |volume=100 |page=212 }}</ref> Because it is so close to the celestial north pole, its [[right ascension]] is changing rapidly due to the [[Axial precession|precession of Earth's axis]], going from 2.5h in AD 2000 to 6h in AD 2100. Twice in each [[sidereal day]] Polaris's [[azimuth]] is true north; the rest of the time it is displaced eastward or westward, and the bearing must be corrected using tables or a [[rule of thumb]]. The best approximation<ref name="kaizad_co_uk" /> is made using the leading edge of the "[[Big Dipper]]" [[Asterism (astronomy)|asterism]] in the constellation Ursa Major. The leading edge (defined by the stars [[Alpha Ursae Majoris|Dubhe]] and [[Beta Ursae Majoris|Merak]]) is referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.

The apparent motion of Polaris towards and, in the future, away from the celestial pole, is due to the [[precession of the equinoxes]].<ref name="Nor">{{cite book | title=Norton's Star Atlas | date=2004 | publisher=Pearson Education | isbn=978-0-13-145164-3 | editor-last=Ridpath | editor-first=Ian | location=New York | page=[https://archive.org/details/nortonsstaratlas00ianr/page/5 5] | 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°.5. | url-access=registration | url=https://archive.org/details/nortonsstaratlas00ianr/page/5 }}</ref> The celestial pole will move away from α UMi after the 21st century, passing close by [[Gamma Cephei]] by about the [[5th millennium#Astronomical events|41st century]], moving towards [[Deneb]] by about the [[10th millennium#Astronomical events|91st century]].{{fact|date=January 2025}}

The celestial pole was close to [[Thuban]] around 2750 BCE,<ref name="Nor" /> and during [[classical antiquity]] it was slightly closer to [[Beta Ursae Minoris|Kochab]] (β UMi) than to Polaris, although still about {{val|10|ul=°}} from either star.<ref>{{cite web | first=Ian | last=Ridpath | url=http://www.ianridpath.com/startales/ursaminor.html#polaris | work=Star Tales | title=Ursa Minor, the Little Bear | date=2018 | access-date=20 August 2016 }}</ref> It was about the same angular distance from β UMi as to α UMi by the end of [[late antiquity]]. The Greek navigator [[Pytheas]] in ca. 320 BC described the celestial pole as devoid of stars. However, as one of the brighter stars close to the celestial pole, Polaris was used for navigation at least from late antiquity, and described as ἀεί φανής (''aei phanēs'') "always visible" by [[Stobaeus]] (5th century), also termed Λύχνος (''Lychnos'') akin to a burner or lamp and would reasonably be described as ''stella polaris'' from about the [[High Middle Ages]] and onwards, both in Greek and Latin. On his first trans-Atlantic voyage in 1492, [[Christopher Columbus]] had to correct for the "circle described by the pole star about the pole".<ref>{{cite book | title=The Life of the Admiral Christopher Columbus by His Son Fredinand | first=Ferdinand | last=Columbus | author-link=Ferdinand Columbus |translator-link=Benjamin Keen | translator1-first=Benjamin | translator1-last=Keen | publication-place=London | publisher=Folio Society | date=1960 | page=74 }}</ref> In [[William Shakespeare|Shakespeare's]] play [[Julius Caesar (play)|Julius Caesar]], written around 1599, Caesar describes himself as being "as constant as the northern star", although in Caesar's time there was no constant northern star. Despite its relative brightness, it is not, as is popularly believed, the brightest star in the sky.<ref>{{Cite news |last=Geary |first=Aidan |date=June 30, 2018 |title=Look up, be patient and 'think about how big the universe is': Expert tips for stargazing this summer |url=https://www.cbc.ca/news/canada/manitoba/stargazing-constellations-how-to-1.4726939 |access-date=June 29, 2024 |work=[[Canadian Broadcasting Corporation]]}}</ref>

Polaris was referenced in the classic [[Nathaniel Bowditch]] maritime navigation book ''[[American Practical Navigator]]'' (1802), where it is listed as one of the [[navigational stars]].<ref name="BowditchAgency2002">{{cite book | title=The American practical navigator : an epitome of navigation | first1=Nathaniel | last1=Bowditch | author2=National Imagery and Mapping Agency | publisher=Paradise Cay Publications | year=2002 | isbn=978-0-939837-54-0 | page=248 | chapter=15 | author1-link=Nathaniel Bowditch | chapter-url=https://books.google.com/books?id=pXjHDnIE_ygC&pg=PR1}}</ref>