Circumpolar Arctic
The Arctic Circle is the most northerly of the abstract five major circles of latitude as shown on maps of the Earth. It marks the northernmost point at which the noon sun is just visible on the northern winter solstice and the southernmost point at which the midnight sun is just visible on the northern summer solstice. 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 21 September 2017, it runs 66°33′46.8″ 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 metres (49 feet) 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 Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the sun can remain continuously above or below the horizon for twenty-four hours; as a result, at least once each year at any location within the Arctic Circle the sun is visible at local midnight, and at least once it is not visible at local noon.
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