Antarctic Circle
The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of the Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. South of the Antarctic Circle, the sun is above the horizon for 24 continuous hours at least once per year (and therefore visible at midnight) and (at least partially) below the horizon for 24 continuous hours at least once per year (and therefore not fully visible at noon); this is also true within the equivalent polar circle in the Northern Hemisphere, the Arctic Circle.
The position of the Antarctic Circle is not fixed; as of 15 January 2017, it runs 66°33′46.5″ south of the Equator. Its latitude depends on the Earth's axial tilt, which fluctuates within a margin of 2° over a 40,000-year period, due to tidal forces resulting from the orbit of the Moon. Consequently, the Antarctic Circle is currently drifting southwards at a speed of about 15 m (49 ft) per year.
The Antarctic Circle is the northernmost latitude in the Southern Hemisphere at which the sun can remain continuously above or partially below the horizon for twenty-four hours; as a result, at least once each year at any location within the Antarctic Circle the sun is visible at local midnight, and at least once it is partially obscured at local noon.
Directly on the Antarctic Circle these events occur, in principle, exactly once per year: at the December and June solstices, respectively. However, because of atmospheric refraction and mirages, 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 southern summer solstice up to about 50 minutes (′) (90 km (56 mi)) north of the Antarctic Circle; similarly, on the day of the southern winter solstice, part of the sun may be seen up to about 50′ south of the Antarctic Circle. That is true at sea level; those limits increase with elevation above sea level, although in mountainous regions there is often no direct view of the true horizon. Mirages on the Antarctic continent tend to be even more spectacular than in Arctic regions, creating, for example, a series of apparent sunsets and sunrises while in reality the sun remains under the horizon.
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