Polar circle

Source: Wikipedia, the free encyclopedia.
For the ship of this name, see HMS Endurance. For the concept in geometry, see Polar circle (geometry).
The north polar circle on a polar projection.
cylindrical projection
.
The Arctic circle in Finland, 1975.
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 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 66°33′49.9″ 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 66°33′49.9″ S.[1] Polar circles are often equated with polar regions of Earth. Due to their inherent 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

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 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 −90 degrees (which refers to the North and South Pole, respectively) minus the

second of arc
(″) in the tilt is equivalent to a change of about 31 metres north or south in the positions of the polar circles on the Earth's surface).

Correspondence to polar night and day

Relationship between Earth's axial tilt (ε) to the tropical and polar circles

The polar circles would almost precisely match the boundaries for the zones where the

Arctic circle, being 80–100 km north of the circle in winter, and 80–100 km south of the circle in summer; the inverse directions apply to the other circle.[2]

See also

Notes

  1. ^ Obliquity of the ecliptic Archived 2017-06-12 at the Wayback Machine
  2. ^ Swedish Astronomic calendar 2003 (or any other year) at the times of the winter and summer solstices, around 22 June and 22 December
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