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The Arctic Circle is a region on Earth that is marked as one of the five major circles of latitude on maps of our planet. It is located between the Arctic to its north and the Northern Temperate Zone to its south. Its counterpart in the Southern Hemisphere is known as the Antarctic Circle. In addition to the Antarctic Circle, the other three major latitude lines are the Tropic of Cancer, the Tropic of Capricorn, and the Equator.
The Arctic Circle is the farthest southern region that experiences polar day and polar night. A polar day is where it is 24 hours of continuous daylight, and a polar night is 24 continuous hours of darkness. North of the Arctic Circle more than one polar day and night occur per year. However, the Arctic Circle only experiences a polar day and night once a year on the June and December solstices respectively. Usually, those days fall on the 21st of each month. Polar day is also known as midnight sun and polar-summer while polar night is also known as darkness at noon or midwinter darkness. The Antarctic Circle also experiences a polar day and a polar night once a year.
Due to the Earth’s shifting axial tilt – a fluctuation of 2° during 40,000 years – which is especially a result of the Moon’s orbit, the Arctic Circle is moving. It is drifting north about 15 kilometers per year. The Earth’s axial tilt is the same thing that causes the different seasons on Earth.
The Arctic Circle passes through seven countries that have a considerable portion of land within the Arctic Circle. The countries are the United States of America, Greenland, Canada, Russia, Norway, Sweden, and Finland. Iceland has a tiny region – less than one square km – inside the Arctic Circle. Some areas within the Arctic Circle include Lapland Province in Finland; Yukon, the Northwest Territories, and part of Nunavut in Canada; Davis Strait in the Atlantic Ocean; the Island of Grimsey, which is part of Iceland; and the Greenland and Norwegian Seas in the Atlantic Ocean. The Arctic Circle is also a popular tourist site. People travel from different parts of the world to experience the region. Many places in the Arctic Circle also offer tours; Alaska has quite a few tours, although you can also find them in other countries. Some of these involve hiking around the Arctic Circle region during the summer.
Universe Today has articles on why Earth has seasons and 10 interesting facts about Earth in case you want to learn more about our planet.
If you are looking for more information, check out what is the Arctic and the Arctic.
Astronomy Cast has an episode on Earth.
Sources:
http://nsidc.org/arcticmet/basics/arctic_definition.html
http://en.wikipedia.org/wiki/Arctic_Circle