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′ 44″ N, and the Antarctic Circle is located at a latitude of 66° 33′ 44″ 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. The exact location of the polar circles differs annually as the rotation axis of the earth shifts due to nutation and precession. Therefore the latitudes noted above are an average of those yearly changes.