Editing Circumpolar star (section)

== 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]].