Arctic Circle

(geodesy) The parallel of latitude 66°32′N (often taken as 66½°N).

The parallel of latitude approximately 66½° (66.55°) north of the Equator, or 23½° from the North Pole. The Arctic Circle has the same angular distance from the Equator as the inclination of the Earth's axis from the plane of the ecliptic. Thus, when the Earth in its orbit is at the Northern Hemisphere summer solstice, June 21, and the North Pole is tilted 23½° toward the Sun, the Sun's rays extend beyond the pole 23½° to the Arctic Circle, giving that parallel 24 h of sunlight. On this same date the Sun's rays at noon will just reach the horizon at the Antarctic Circle, 66½° south. The highest altitude of the noon Sun at the Arctic Circle is on June 21, when it is 47° above the horizon.

At the Arctic Circle the Sun remains above the horizon continuously only 24 h at the longest period. However, with twilight considered, it remains daylight or twilight continuously for about 5 months. Twilight can be considered to last until the Sun drops 18° below the horizon. See also Mathematical geography; Solstice.