Arctic Circle

Arctic Circle, a parallel of latitude 66° 32' N; similarly, the Antarctic Circle is the parallel of latitude 66° 32' S. Between these circles the Sun rises and sets daily. North of the Arctic Circle the sun remains above the horizon at midnight at midsummer and never rises at midwinter. Since light rays are bent by the Earth's atmosphere, the sun can be seen when it is slightly below the horizon. Thus, the midwinter sun can be seen at places slightly north of the circle.

The number of days the sun stays above(or below) the horizon increases the farther north one goes until, at the pole, the sun never sets for 6 months and never rises during the other 6. These effects occur because the Earth's axis of rotation is tilted relative to the plane of its orbit around the sun.

During the northern winter, when the Northern Hemisphere is tilted away from the sun, the Earth's curvature creates an area of permanent shadow centred on the North Pole. This area starts to form at the fall equinox (September 22), grows to a maximum at midwinter, then decreases, vanishing by the spring equinox (March 21). For the remainder of the year, when the northern hemisphere is tilted towards the sun, an area of permanent sunlight centres on the pole. Because the tilt of the Earth's axis is about 23.5°, the Arctic and Antarctic circles lie at latitudes of about 66.5°.

The Arctic Circle is not a climatic boundary. Trees grow north of it in the Mackenzie Delta; in Nouveau-Québec the Treeline is 1000 km farther S. Inuvik, NWT, is the only moderately large Canadian settlement lying north of the circle.