A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of 66°33′46.1″ N, and the Antarctic Circle is located at a latitude of 66°33′46.1″ S.

Areas between each polar circle and its associated pole (North Pole or South Pole), known geographically as the frigid zones, would theoretically experience at least one 24-hour period when the sun is continuously above the horizon and at least one 24-hour period when the sun is continuously below the horizon annually. However, due to atmospheric refraction and the Sun being an extended object rather than a point source, the continuous daylight area is somewhat extended while the continuous darkness area is somewhat reduced.