The position of the Antarctic Circle is not fixed; as of 25 June 2018, it runs 66°33′47.2″ 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.
There is no permanent human population south of the Antarctic Circle, but there are several research stations in Antarctica operated by various nations that are inhabited by teams of scientists who rotate on a seasonal basis. In previous centuries some semi-permanent whaling stations were established on the continent, and some whalers would live there for a year or more. At least three children have been born in Antarctica, albeit in stations north of the Antarctic Circle.
The Antarctic Circle is roughly 17,662 kilometres (10,975 mi) long. The area south of the Circle is about 20,000,000 km2 (7,700,000 sq mi) and covers roughly 4% of Earth's surface. The continent of Antarctica covers much of the area within the Antarctic Circle.