ARCTIC CIRCLE

parallel, or line of latitude around the Earth, at approximately 6630 N. Because of the Earth's inclination of about 23 1/2 to the vertical, it marks the southern limit of the area within which, for one day or more each year, the Sun does not set (about June 21) or rise (about December 21). The length of continuous day or night increases northward from one day on the Arctic Circle to six months at the North Pole. The Antarctic Circle is the southern counterpart of the Arctic Circle, where on any given date conditions of daylight or darkness are exactly opposite.