Editing Circumpolar star

{{short description|Star that never sets due to its apparent proximity to a celestial pole}}
{{More citations needed|date=May 2018}}
[[File:Circumpolar AZ81.jpg|thumb|Circumpolar [[star trail]]s in a [[long-exposure photo]] of several hours. The stars near the celestial pole leave shorter trails with the long exposure.]]
A '''circumpolar star''' is a [[star]] that, as viewed from a given [[latitude]] on Earth, never sets below the [[horizon]] due to its apparent proximity to one of the [[celestial pole]]s. Circumpolar stars are therefore visible from said location toward the nearest pole for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the [[Sun]]'s glare). Others are called ''seasonal'' stars.

All circumpolar stars lie within a '''circumpolar circle''' whose size is determined by the observer's latitude. Specifically, the angular measure of the radius of this circle equals the observer's latitude. The closer the observer is to the North or [[South Pole]], the larger its circumpolar circle. 

Before the definition of the [[Arctic]] was formalized as the region north of the [[Arctic Circle]] which experiences the [[midnight sun]], it more broadly meant those places where the 'bear' constellations ([[Ursa Major]], the Great Bear, and [[Ursa Minor]], the Little Bear) were high in the sky. Thus the word 'Arctic' is derived from the [[Ancient Greek language|Greek]] {{lang|grc|ἀρκτικός}} ({{lang|grc-Latn|arktikos}}), 'bearish', from {{lang|grc|ἄρκτος}} ({{lang|grc-Latn|arktos}}), 'bear'.

== Explanation ==
[[File:Circumpolar diagram5.png|left|thumb|upright=1.6|Geometric diagram proving that the angle between the North Celestial Pole and the observer's horizon, i.e., the angle that spans the radius of the circumpolar circle, is equal to the observer's latitude.]]
As [[Earth's rotation|Earth rotates]] daily on its axis, the stars appear to move in circular paths around one of the [[celestial pole]]s (the north celestial pole for observers in the [[Northern Hemisphere]], or the south celestial pole for observers in the [[Southern Hemisphere]]). Stars far from a celestial pole appear to rotate in large circles; stars located very close to a celestial pole rotate in small circles and hence hardly seem to engage in any [[diurnal motion]] at all. Depending on the observer's latitude on Earth, some stars &ndash; the circumpolar ones &ndash; are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the [[horizon]] for some portion of their daily circular path (and others remain permanently below the horizon).

The circumpolar stars appear to lie within a circle that is centered at the celestial pole and [[tangent]]ial to the horizon. At the Earth's [[North Pole]], the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the [[Northern Celestial Hemisphere]]) are circumpolar.<ref name=Ridpath2006>{{cite book| title=Eyewitness Companions: Astronomy| first1=Ian | last1=Ridpath | year=2006| publisher=Penguin | isbn=0756648459 | page=148| url=https://books.google.com/books?id=OEZ7n1YKMlMC&pg=PA148 }}</ref> As one travels south, the north [[celestial pole]] moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily "[[orbit]]", and the circle containing the remaining circumpolar stars becomes increasingly small. At the [[Equator]], this circle [[vanishing point|vanishes to a single point]] &ndash; the celestial pole itself &ndash; which lies on the horizon, and so all of the stars capable of being circumpolar are for half of every 24 hour period below the horizon. There, the pole star itself will only be made out from a place of sufficient height.

As one travels south of the Equator, the opposite happens. The south celestial pole appears increasingly high in the sky, and all the stars lying within an increasingly large circle centred on that pole become circumpolar about it. This continues until one reaches the Earth's [[South Pole]] where, once again, all visible stars are circumpolar.

The [[celestial pole|celestial north pole]] is located very close (less than 1° away) to the [[pole star]] (Polaris or North Star), so from the Northern Hemisphere, all circumpolar stars appear to move around Polaris. Polaris itself remains almost stationary, always at the north (i.e. [[azimuth]] of 0°), and always at the same [[altitude]] (angle from the horizon), equal to the observer's [[latitude]]. These are then classified into quadrants.

Polaris always has an [[azimuth]] equal to [[true north|zero]]. The pole's [[altitude]] for a given latitude Ø is fixed, and its value is given by Ø. All stars with a [[declination]] less than 90° – Ø are not circumpolar.<ref name=Hannu2007>{{cite book| title=Fundamental Astronomy| editor1-first=Hannu | editor1-last=Karttunen| editor2-first=Pekka | editor2-last=Kröger| editor3-first=Heikki | editor3-last=Oja| editor4-first=Markku | editor4-last=Poutanen| editor5-first=Karl Johan | editor5-last=Donner| publisher=[[Springer Science & Business Media]]| page=19 | edition=5th | year=2007 | isbn=978-3540341444| url=https://books.google.com/books?id=DjeVdb0sLEAC&pg=PA19}}</ref>

== Definition ==
[[File:Star trails over the ESO 3.6-metre telescope edited.jpg|thumb|Circumpolar [[star trail]]s captured with an [[long-exposure photography|extended exposure]]|upright=1.5]]
[[File:Circumpolar Star Diagram.png|thumb|Diagram of circumpolar stars assuming an observer is at latitude +19°.]]
Whether a star is circumpolar depends upon the observer's [[latitude]].<ref name="NortonSA">{{cite book |last=Norton |first=A. P. |title=Norton's 2000.0 Star Atlas and Reference Handbook |publisher=Longman Scientific and Technical |year=1986 |pages=39–40}}</ref> Since the [[horizontal coordinate system|altitude]] of the north or [[Celestial pole#Finding the south celestial pole|south celestial pole]] (whichever is visible) equals the [[absolute value]] of the observer's latitude,<ref name=NortonSA /> any star whose [[angular distance]] from the visible [[celestial pole]] is less than the absolute latitude will be circumpolar. For example, if the observer's latitude is 50°&nbsp;N, any star will be circumpolar if it is less than 50° from the north celestial pole. If the observer's latitude is 35°&nbsp;S, then all stars within 35° of the south celestial pole are circumpolar. Stars on the [[celestial equator]] are not circumpolar when observed from any latitude in either hemisphere of the Earth.

Whether a given star is circumpolar at the observer's latitude ({{mvar|θ}}) may be calculated in terms of the star's [[declination]] ({{mvar|δ}}). The star is circumpolar if {{math|''θ'' + ''δ''}} is greater than +90° (observer in [[Northern Hemisphere]]), or {{math|''θ'' + ''δ''}} is less than −90° (observer in [[Southern Hemisphere]]). Similarly, the star will never rise above the local horizon if {{math|''δ'' − ''θ''}} is less than −90° (observer in Northern Hemisphere), or {{math|''δ'' − ''θ''}} is greater than +90° (observer in Southern Hemisphere). Thus, [[Canopus]] is invisible from [[San Francisco]] and [[London]], and marginally visible from [[Seville]], [[Seoul]], and [[Nashville, Tennessee|Nashville]].

Some stars within the far northern [[constellation]] (such as [[Cassiopeia (constellation)|Cassiopeia]], [[Cepheus (constellation)|Cepheus]], [[Ursa Major]], and [[Ursa Minor]]) roughly north of the [[Tropic of Cancer]] (23°&nbsp;26′&nbsp;N) will be circumpolar stars, which never rise or set.<ref name=NortonSA />

For British Isles observers, for example, the first magnitude stars [[Capella (star)|Capella]] (declination +45°&nbsp;59′) and [[Deneb]] (+45°&nbsp;16′) do not set from anywhere in the country. [[Vega]] (+38°&nbsp;47′) is technically circumpolar north of latitude 51°&nbsp;13′&nbsp;N (just south of [[London]]); taking [[atmospheric refraction]] into account, it will probably only be seen to set at sea level from [[Cornwall]].

Stars in the far southern constellations (such as [[Crux (constellation)|Crux]], [[Musca (constellation)|Musca]], and [[Hydrus (constellation)|Hydrus]]) roughly south of the [[Tropic of Capricorn]] (23°&nbsp;26′&nbsp;S) are circumpolar to typical points of observation beyond that tropic.<ref name=NortonSA />

Stars (and constellations) that are circumpolar in one hemisphere are always invisible at the same latitude (or higher) of the opposite hemisphere, and these never rise above the horizon. For example, the southern star [[Acrux]] is invisible from most of the [[contiguous United States]]; likewise, the seven stars of the northern [[Big Dipper]] asterism are invisible from most of the [[Patagonia]] region of [[South America]].

==Constellations==
A '''circumpolar constellation''' is a [[constellation]] (group of [[star]]s) that never sets below the [[horizon]], as viewed from a location on [[Earth]]. As viewed from the [[North Pole]], all fully visible constellations north of the [[celestial equator]] are circumpolar, and likewise for constellations south of the celestial equator as viewed from the [[South Pole]]. As viewed from the [[Equator]], there are no circumpolar constellations. As viewed from mid-northern latitudes (40–50° N), circumpolar constellations may include [[Ursa Major]], [[Ursa Minor]], [[Draco (constellation)|Draco]], [[Cepheus (constellation)|Cepheus]], [[Cassiopeia (constellation)|Cassiopeia]], and the less-known [[Camelopardalis (constellation)|Camelopardalis]], [[Lynx (constellation)|Lynx]] and [[Lacerta (constellation)|Lacerta]].<ref name=Young1897>{{citation| title=Uranography: A Brief Description of the Constellations Visible in the United States, with Star-maps, and Lists of Objects Observable with a Small Telescope| first1=Charles Augustus | last1=Young| publisher=Ginn | year=1897 | pages=9–14| url=https://books.google.com/books?id=tylLAAAAIAAJ&pg=PA9| postscript=. }}</ref>

== See also ==
* [[Celestial pole]]
* [[Celestial sphere]]
* [[Magellan expedition]]
* [[Pole star]]
* [[Voyages of Christopher Columbus]]

==References==
{{Reflist}}
*"Circumpolar Stars." Web. 7 Jan. 2015. http://www.astronomygcse.co.uk/AstroGCSE/New{{Dead link|date=November 2019 |bot=InternetArchiveBot |fix-attempted=yes }} Site/Topic 3/circumpolar_stars.htm.

==External links==
{{Commons category|Circumpolar stars}}
* [https://web.archive.org/web/20071001050942/http://www.bartleby.com/65/ci/circumpo.html Bartleby.com: Circumpolar Star]
* [https://web.archive.org/web/20070302120632/http://astro2.byu.edu/sdb/Astrophotos/GuidingLights.html North Circumpolar Star Trails]
* [https://web.archive.org/web/20070122141316/http://astro2.byu.edu/sdb/Astrophotos/MagesticTrails.html South Circumpolar Star Trails]
* [https://web.archive.org/web/20120212074531/http://adwheelerphotography.com/2012/01/03/shoot-star-trails-the-easy-way/ Star Trails Photography Tutorial]

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