The Antarctic Circle is one of the five major circles (or parallels) of latitude that mark maps of the Earth. For 2012, it is the parallel of latitude that runs 66° 33′ 44″ (or 66.5622°) south of the Equator. The area south of the Antarctic Circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. The equivalent line of latitude in the northern hemisphere is the Arctic Circle. The Antarctic Circle is the northernmost latitude in the Southern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the December solstice and June solstice respectively). 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 below the horizon for 24 continuous hours at least once per year. On the Antarctic Circle those events occur, in principle, exactly once per year, at the December and June solstices, respectively. It would therefore seem that the Antarctic Circle marks the northern extremity of the southern hemisphere polar day (24-hour sunlit day, often referred to as the midnight sun) and polar night (24-hour sunless night). In fact, because of atmospheric refraction, 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 southern summer solstice up to about 50′ (90 km (56 mi)) north 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. Every place south of the Antarctic Circle experiences a period of twenty-four hours' continuous daylight at least once per year, and a period of twenty-four hours' continuous night time at least once per year. That is to say, there is at least one whole day during which the sun does not set, and at least one whole day during which the sun does not rise. On the Antarctic Circle these events occur, in principle, exactly once per year, at the December and June solstices respectively. This happens because the Earth's axis is tilted, by approximately 23.5 degrees, relative to ecliptic (the plane of the Earth's orbit around the sun). At the southern winter solstice, the southern hemisphere is tilted away from the Sun to its maximum extent, and the region of permanent darkness reaches its northern limit; at the southern summer solstice, the southern hemisphere is tilted towards the Sun to its maximum extent, and the region of permanent sunlight reaches its northern limit. Due to gradual changes in the tilt of the Earth's axis, the Antarctic Circle is slowly moving. See circles of latitude. The area south of the Antarctic Circle is about 40,000,000 km2 (15,000,000 sq mi) and covers 8% of the Earth.