The Arctic Circle marks thesouthernmost latitude for which, at thewinter solstice in theNorthern Hemisphere (in December), theSun does not rise at all. Likewise the Antarctic Circle marks thenorthernmost latitude for which, at thesummer solstice in the Southern Hemisphere (also in December), the Sun does not set. These phenomena are referred to aspolar night andmidnight sun respectively, and the closer to the respective pole one goes, the longer that situation persists. For example, in the Russian port city ofMurmansk (three degrees north of the Arctic Circle) the Sun stays below the horizon for 20 days before and after the winter solstice, and above the horizon for 20 days before and after the summer solstice.[2][3][4]
The positions of the Arctic and Antarctic Circles vary slightly from year to year. Currently the Arctic Circle is 66°33′50.7″ north of theEquator.[5] Its latitude depends on Earth'saxial tilt, whichfluctuates by a margin of some 2° over a 41,000-year period due totidal forces resulting from theorbit of the Moon.[6] Consequently, in the current epoch the Arctic Circle isdrifting toward the North Pole, while the Antarctic Circle drifts towards the South Pole, each at a speed of about 14.5 m (48 ft) per year.
Relationship of Earth's axial tilt (ε) to the tropical and polar circles
The Arctic Circle is the southernmost latitude in theNorthern Hemisphere at which the centre of 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 centre of the Sun isvisible at local midnight, and at least once the centre isnot visible at local noon.[9]
Directly on the Arctic Circle these events occur, in principle, exactly once per year: at theJune andDecembersolstices, respectively. However, because ofatmospheric refraction andmirages, and also because the sun appears as a disk and not a point, part of the midnight sun is visible, on the night of the northernsummer solstice, at a latitude of about 50minutes of arc (′) (90 km (56 mi)) south of the Arctic Circle. Similarly, on the day of the northernwinter solstice, part of the Sun may be seen up to about 50′ north of the Arctic Circle. That is true atsea level; those limits increase withelevation above sea level, although in mountainous regions there is often no direct view of the true horizon.
The largest communities north of the Arctic Circle are situated in Russia, Norway, and Sweden:Murmansk (population 295,374) andNorilsk (178,018) in Russia;Tromsø (75,638) in Norway,Vorkuta (58,133) in Russia,Bodø (52,357) andHarstad (24,703) in Norway; andKiruna, Sweden (22,841). In Finland, the largest settlement in the immediate vicinity of the Arctic Circle isRovaniemi (62,667), lying 6 km (4 mi) south of the line.Salekhard (51,186) in Russia is the only city in the world located directly on the Arctic Circle.[10]
In contrast, the largest North American community north of the Arctic Circle,Sisimiut (Greenland), has approximately 5,600 inhabitants. In the United States,Utqiagvik, Alaska (formerly known as Barrow) is the largest settlement north of the Arctic Circle with about 5,000 inhabitants. The largest such community in Canada isInuvik in theNorthwest Territories, with 3,137 inhabitants.
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The Arctic Circle is roughly 16,000 km (9,900 mi) in circumference.[11] The area north of the Circle is about 20,000,000 km2 (7,700,000 sq mi) and covers roughly 4% of Earth's surface.[12]
The climate north of the Arctic Circle is generally cold, but the coastal areas of Norway have a generally mild climate as a result of theGulf Stream, which makes the ports of northern Norway and northwest Russia ice-free all year long. In the interior, summers can be quite warm, while winters are extremely cold. For example, summer temperatures inNorilsk, Russia will sometimes reach as high as 30 °C (86 °F), while the winter temperatures frequently fall below −50 °C (−58 °F).
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