Polar Circles, two small circles or parallels of latitude, situated so that the arc of a meridian included between each of them and the nearest pole of the earth measures the angle of inclination of*the earth's axis to the ecliptic. This angle is about 23° 28', and the polar circles are therefore in about 66° 32' N". and S. latitude respectively. The northern is called the Arctic, the southern the Antarctic circle. According to the common division of geographers, the former is the boundary between the north frigid and the north temperate, the latter between the south frigid and the south temperate zones. They are also generally considered as the respective boundaries of the Arctic and Antarctic oceans. The portions of the earth enclosed by the polar circles are the only regions in which the day or the night is ever more than 24 hours long. On the circles themselves the greatest length of day (or of night in its alternation) is almost exactly 24 hours, and the length increases as the poles are approached until the six-months day or night of the pole itself is reached.

The regions within the Arctic circle have been in some degree explored (see Arctic Discovery, and Polar Seas), while those within the Antarctic circle are almost unknown. (See Antarctic Discovery).