Antarctic Circle

The Antarctic Circle is the southernmost of Earth’s five major circles of latitude, marking the boundary of the Antarctic region to its south and the Southern Temperate Zone to its north. Within this circle, the sun remains above the horizon for a full twenty-four hours at least once each year, making it visible at solar midnight; conversely, the sun’s center also stays below the horizon for a continuous twenty-four hours at least once annually, so it is not seen at solar noon. This same phenomenon occurs within the Arctic Circle in the Northern Hemisphere.

The circle’s position is not fixed. Its latitude is determined by Earth’s axial tilt, which fluctuates by more than two degrees over a 41,000-year cycle due to tidal forces from the Moon’s orbit. Currently, the circle runs south of the Equator, though this value does not account for astronomical nutation—a slight wobble that can shift the circle by up to ten arcseconds.

As a result, the Antarctic Circle is drifting southward at roughly 14.5 meters per year. Directly on the circle, the midnight sun and polar night occur exactly once per year, at the December and June solstices respectively. However, atmospheric refraction, mirages, and the sun’s apparent disk mean that part of the midnight sun can be seen up to about 50 minutes of latitude (roughly 90 kilometers) north of the circle on the summer solstice, and part of the sun may appear up to 50 minutes south of the circle on the winter solstice. These limits increase with elevation.

No permanent human population lives south of the Antarctic Circle, though several research stations operate there with rotating teams of scientists. The circle’s circumference is roughly 16,000 kilometers, and the area south of it covers about 20 million square kilometers—approximately four percent of Earth’s surface. Most of the Antarctic continent lies within this circle.