Editing Circumpolar star (section)

== 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>