Editing Polar circle (section)

{{Short description|Circle of latitude}}
{{About||the ship of this name|HMS Endurance|the concept in geometry|Polar circle (geometry)}}
{{More citations needed|date=December 2016}}

[[File:Arctic circle.svg|thumb|The north polar circle on a [[Azimuthal equidistant projection|polar projection]].]]
[[File:World map with polar circles.svg|thumb|The polar circle as lines on a modified [[cylindrical projection]].]]
[[File:Polarkreis Finnland 1975.jpg|thumb|The Arctic Circle in [[Finland]], 1975.]]
[[File:Polarkreis - zwischen Narvik & Mo i Rana Norwegen.JPG|thumb|The Arctic Circle in Norway at [[Saltfjellet]] mountain [[plateau]] in July 2003.]]

A '''polar circle''' is a geographic term for a conditional circular line (arc) referring either to the [[Arctic Circle]] or the [[Antarctic Circle]]. These are two of the keynote [[Circle of latitude|circles of latitude]] (parallels). On [[Earth]], the Arctic Circle is currently drifting northwards at a speed of about 14.5 m per year and is now at a mean [[latitude]] (i.e. without taking into account the [[astronomical nutation]]) of {{Circle of latitude|Polar}}&nbsp;N; the Antarctic Circle is currently drifting southwards at a speed of about 14.5 m per year and is now at a mean latitude of {{Circle of latitude|Polar}}&nbsp;S.<ref>[http://www.neoprogrammics.com/obliquity_of_the_ecliptic/ ''Obliquity of the ecliptic''] {{webarchive|url=https://web.archive.org/web/20170612041431/http://www.neoprogrammics.com/obliquity_of_the_ecliptic/ |date=2017-06-12 }}</ref> Polar circles are often equated with [[polar regions of Earth]]. Due to their inherent [[Polar climate|climate environment]], the bulk of the Arctic Circle, much of which is sea, is sparsely settled whereas this applies to all of [[Antarctica]] which is mainly land and sheltered ice shelves.

If Earth had no atmosphere, then both polar circles (arcs) would see at least a day a year when the center of the Sun is continuously above the [[horizon]] ''and'' at least a day a year when it is always below the horizon &ndash; a polar day and a polar night as is the case for longer, ''within'' the circles. Up to and including the associated poles ([[North Pole|North]] and [[South Pole|South]]), known geographically as the [[frigid (geography)|frigid zones]], such duration extends up to half of the year, namely, close to the poles. Instead, [[atmospheric refraction]] and the Sun's light reaching the planet as an [[Angular diameter|extended object]] rather than a [[point source]] means that just within each circle the Earth's surface does not experience any proper polar night, 24 hours where the sun does not rise. By these same two factors, just outward of each circle still experiences a polar day (a day in which the sun does not fully set).

The latitude of the polar circles is + or &minus;90 degrees (which refers to the North and South Pole, respectively) minus the [[axial tilt]] (that is, of the Earth's axis of daily rotation relative to the [[ecliptic]], the plane of the Earth's orbit).  This predominant, average tilt of the Earth varies slightly, a phenomenon described as nutation.  Therefore, the latitudes noted above are calculated by averaging values of tilt observed over many years.  The axial tilt also exhibits long-term variations as described in the reference article (a difference of 1 [[arcsecond|second of arc]] (″) in the tilt is equivalent to a change of about 31&nbsp;metres north or south in the positions of the polar circles on the Earth's surface).