Antarctic Circle

The Antarctic Circle is the southernmost of Earth’s five main latitude lines. South of it lies the Antarctic region; just north is the Southern Temperate Zone. Inside the circle, the Sun stays above the horizon for a full 24 hours at least once a year (visible at midnight) and stays below the horizon for a full 24 hours at least once a year (invisible at noon). The same happens inside the Arctic Circle in the north.

The circle’s location shifts over time. Currently, ignoring nutation, it sits at 66°33′51.0″ south of the Equator. That figure is slightly off because it doesn’t account for astronomical nutation, which can alter it by up to 10″.

The latitude depends on Earth’s axial tilt, which wobbles 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 drifting southward at roughly 14.5 meters (48 feet) per year. The Antarctic Circle marks the northernmost point in the Southern Hemisphere where the Sun’s center can stay above the horizon for 24 hours straight. So, anywhere inside the circle, the Sun’s center is visible at local midnight at least once a year, and hidden at local noon at least once a year.

Right on the circle, these events happen exactly once per year—at the December and June solstices. But because of atmospheric refraction, mirages, and the Sun’s disk shape (not a point), part of the midnight sun can be seen up to about 50 minutes (90 km or 56 mi) north of the circle on the summer solstice night. Similarly, part of the Sun may appear up to 50′ south of the circle on the winter solstice day. These limits apply at sea level; they increase with elevation, though mountains often block a direct view of the true horizon.

Human habitation

Mirages in Antarctica are often more dramatic than in the Arctic, sometimes creating a series of fake sunsets and sunrises while the Sun actually 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. In past centuries, semi-permanent whaling stations existed 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 km² (7,700,000 sq mi), or roughly 4% of Earth’s surface. Most of Antarctica lies inside the circle.