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Arctic Circle
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The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. It is the parallel of latitude that (as of 2000) runs 66° 33' 39? (or 66.56083°) north of the Equator. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone. The equivalent latitude in the southern hemisphere is called the Antarctic Circle.
The Arctic Circle marks the southern extremity of the polar day (24 hour sunlit day, often referred to as the "midnight sun") and polar night (24 hour sunless night). North of the Arctic Circle, the sun is above the horizon for 24 continuous hours at least once per year, and below the horizon for 24 continuous hours at least once per year. On the Arctic Circle these events occur, in principle, exactly once per year, at the June and December solstices respectively. It is called the Arctic because it corresponds to the southernmost point of the Constellation Ursa Major (the Great Bear or Megale Arktos in Greek).
In fact, because of atmospheric refraction and because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the summer solstice up to about 50' (90 km) south of the Arctic Circle; similarly, on the day of the winter solstice part of the sun may be seen up to about 50' north of the Arctic Circle. This is true at sea level; these limits increase with elevation above sea level, although in mountainous regions there is often no direct view of the horizon.
The position of the Arctic Circle is not fixed, but varies in a complex manner over time; see circles of latitude for information.
See also
External links- —Historical essay about early expeditions to the Canadian Arctic, illustrated with maps, photographs and drawings
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