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The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone. The equivalent polar circle in the Southern Hemisphere is called the Antarctic Circle.
The Arctic Circle is the southernmost latitude in the Northern Hemisphere at which the sun can remain continuously above or below the horizon for 24 hours (at the June solstice and December solstice respectively). North of the Arctic Circle, the sun is above the horizon for 24 continuous hours at least once per year (and therefore visible at midnight) and below the horizon for 24 continuous hours at least once per year. On the Arctic Circle those events occur, in principle, exactly once per year, at the June and December solstices, respectively.
The Arctic Circle appears to mark the southern extremity of the northern hemisphere polar day (24-hour sunlit day, often referred to as the midnight sun) and polar night (24-hour sunless night). But, because of atmospheric refraction and mirages, and because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the northern summer solstice up to about 50′ (90 km (56 mi)) south of the Arctic Circle; similarly, on the day of the northern winter solstice, part of the sun may be seen up to about 50′ north of the Arctic Circle. That is true at sea level; those limits increase with elevation above sea level although in mountainous regions, there is often no direct view of the true horizon.
The position of the Arctic Circle is not fixed. It directly depends on the Earth's axial tilt, which fluctuates within a margin of 2° over a 40,000-year period, notably due to tidal forces resulting from the orbit of the Moon. The Arctic Circle is currently drifting northwards at a speed of about 15 m (49 ft) per year; see Circle of latitude for more information.
Relatively few people live north of the Arctic Circle due to the severe climate. Areas have been settled for thousands of years by indigenous peoples. Tens of thousands of years ago, waves of people migrated from east Asia (Siberia) across the Bering Straits into North America and gradually eastward to settle. Much later, in the historic period, there has been migration into some Arctic areas by Europeans and other immigrants.
Rovaniemi (in Finland), which lies slightly south of the line, has a population of approximately 60,000, and is the largest settlement in the immediate vicinity of the Arctic Circle. Among the people of the Arctic, the Norwegians have the easiest climate, with most ports in North Norway remaining ice-free all the year as a result of the Gulf Stream. Tromsø (in Norway) has about 69,000 inhabitants and Bodø 47,000.
In contrast, the largest North American community north of the circle, Sisimiut (Greenland), has approximately 5,000 inhabitants. Of the Canadian and United States Arctic communities, Barrow, Alaska is the largest settlement with circa 4,000 inhabitants.
The Arctic Circle passes through the Arctic Ocean, the Scandinavian Peninsula, North Asia, Northern America and Greenland. The land on the Arctic Circle is divided among eight countries: Norway, Sweden, Finland, Russia, the United States (Alaska), Canada, Denmark (Greenland), and Iceland (where it passes through the small offshore island of Grímsey).
The area north of the Arctic Circle is about 20,000,000 km2 (7,700,000 sq mi) and covers 4% of the Earth.
Starting at the Prime Meridian and heading eastwards, the Arctic Circle passes through:
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