Antarctic Circle

The Antarctic Circle is the southernmost of Earth’s five main latitude lines. Land and sea south of this circle make up the Antarctic region, while the area just north of it is the Southern Temperate Zone. Its location shifts over time; currently, ignoring the slight wobble called nutation, it sits at 66°33′51.0″ south of the Equator. That number isn’t exact because nutation—which can shift the line by up to 10 arcseconds—isn’t factored in.

Midnight sun and polar night

The circle’s latitude depends on Earth’s axial tilt, which varies by more than 2° over a 41,000-year cycle due to tidal forces from the Moon’s orbit. As a result, the Antarctic Circle is slowly moving south at about 14.5 meters (48 feet) each year. Within the Antarctic Circle, the Sun stays above the horizon for a full 24 hours at least once a year—meaning it’s visible at midnight—and also stays below the horizon for a full 24 hours at least once a year, so it’s not seen at noon.

The same happens inside the Arctic Circle, its northern counterpart. Right on the circle itself, these events occur exactly once per year: the midnight sun at the December solstice and the polar night at the June solstice. But because of atmospheric refraction, mirages, and the fact that the Sun appears as a disk rather than a point, the midnight sun can be glimpsed up to about 50 arcminutes (roughly 90 kilometers or 56 miles) north of the circle on the summer solstice.

Similarly, part of the Sun may be visible up to 50 arcminutes south of the circle on the winter solstice. These limits apply at sea level; higher elevations extend them, though mountains often block a clear view of the horizon. Mirages in Antarctica are often more dramatic than in the Arctic, sometimes creating a series of false sunsets and sunrises while the Sun stays below the horizon. No one lives permanently south of the Antarctic Circle, but several nations run research stations there, staffed by rotating teams of scientists.

Human habitation

In past centuries, semi-permanent whaling stations were set up on the continent, and some whalers stayed for a year or more. At least eleven children have been born in Antarctica, though all at stations north of the circle. The Antarctic Circle’s circumference is roughly 16,000 kilometers (9,900 miles). The area south of it covers about 20,000,000 square kilometers (7,700,000 square miles), or roughly 4% of Earth’s surface.

Sites along the Circle

Most of Antarctica lies within the circle. Starting at the prime meridian and moving east, the circle passes through various points ( as a table). Related concepts include the Antarctic Convergence, the Tropic of Cancer, and the Tropic of Capricorn.