Arctic circle
The Arctic Circle is the most northerly of the five major circles of latitude that mark maps of the Earth. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone. North of the Arctic Circle, the sun is above the horizon for twenty-four continuous hours at least once per year (and therefore visible at midnight) and below the horizon for twenty-four continuous hours at least once per year (and therefore not visible at noon); this is also true within the equivalent polar circle in the Southern Hemisphere, the Antarctic Circle.
The position of the Arctic Circle is not fixed; as of 17 January 2017, it runs 66°33′46.5″ north of the Equator. Its latitude depends on the Earth's axial tilt, which fluctuates within a margin of 2° over a 40,000-year period, due to tidal forces resulting from the orbit of the Moon. Consequently, the Arctic Circle is currently drifting northwards at a speed of about 15 m (49 ft) per year.
The word "arctic" comes from the Greek word ἀρκτικός (arktikos: "near the Bear, northern") and that from the word ἄρκτος (arktos: "bear"). The name refers either to the constellation Ursa Major, the "Great Bear", which is prominent in the northern portion of the celestial sphere, or to the constellation Ursa Minor, the "Little Bear", which contains Polaris, the Pole star, also known as the North Star.
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