Antarctic Circle, imaginary circle on the surface of the earth at 661/2°S lat., i.e., 231/2° north of the South Pole. It marks the southernmost point at which the sun can be seen at the winter solstice (about June 22) and the northernmost point of the southern polar regions at which the midnight sun is visible.

Parallel of latitude approximately 66°30' south of the Equator that circumscribes the southern frigid zone. It marks the northern limit of the area within which, for one day or more each year, the sun does not set or rise. The length of continuous day or night increases southward from the Antarctic Circle, mounting to six months at the South Pole.

Learn more about Antarctic Circle with a free trial on Britannica.com.