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The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth at about 66° 34' N. Its southern equivalent is the Antarctic Circle.

The Arctic Circle marks the southernmost latitude at which, on the winter solstice (which is the shortest day of the year) in the Northern Hemisphere, the Sun will not rise all day, and on the Northern Hemisphere's summer solstice (which is the longest day of the year), the Sun will not set. These phenomena are referred to as polar night and midnight sun respectively, and the further north one progresses, the more pronounced these effects become. For example, in the Russian port city of Murmansk, three degrees above the Arctic Circle, the Sun does not rise above the horizon for 40 successive days in midwinter.

The position of the Arctic Circle is not fixed and currently runs 66°33′50.1″ north of the Equator. Its latitude depends on the Earth's axial tilt, which fluctuates within a margin of more than 2° over a 41,000-year period, owing to tidal forces resulting from the orbit of the Moon. Consequently, the Arctic Circle is currently drifting northwards at a speed of about 14.5 m (48 ft) per year.