Arctic Circle

Arctic Circle, parallel of latitude on the surface of the Earth at 66°33’ N. It marks the southern limit of the area in which the Sun does not rise on the winter solstice (about December 21) or set on the summer solstice (about June 21). Periods of continuous day or night increase from one day at the Arctic Circle to six months at the North Pole. The latitude of the Arctic Circle is determined by the angle between the Earth’s axis of rotation and a line perpendicular to the plane of the Earth's orbit around the Sun. This angle has a mean value of 23°26’21.448”. During a period of 18.6 years, this angle varies between plus or minus 9”. The latitude of the Antarctic Circle is the complement of this angle and therefore its precise latitudinal value varies between 66°33’29.552” N and 66°33’47.552” N.