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I heard today that the Arctic Circle, along with it's siblings the Antarctic Circle and the Tropics of Cancer and Capricorn, are different from year to year. Is that true? Can you please explain how/why this is so? I'd always thought they were defined by particular latitudinal parallels. e.g. 66°N or S etc.

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This is geography-related, but not a GIS question, shouldn't it be a community wiki? –  Petr Krebs Oct 14 '10 at 20:37
    
When making a map that has the Arctic Circle on it I need to put it in the right place. Though now I see I didn't ask "where is it, exactly?". –  matt wilkie Oct 14 '10 at 21:49
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up vote 3 down vote accepted

I had no idea it moved either!

http://en.wikipedia.org/wiki/Arctic_Circle

The position of the Arctic Circle is not fixed, but directly depends on the Earth's axial tilt, which fluctuates within a margin of 2° over a 40,000 year period,[2] 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.

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Not only the Arctic Circle, but also three other major circles of latitude: the Antarctic Circle, the Tropic of Cancer, and the Tropic of Capricorn. –  Jaime Soto Jan 3 '11 at 22:15
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Yes - I believe this is true. The positions of the Great Circles are not fixed. Their exact locations are relative to the Earth's axial tilt (obliquity). The axial tilt fluctuates within a margin of about 2° over a 40,000 year period. If it weren't for the Moon, this degree of fluctuation would be even greater. I've seen latitude values from different Epochs, but not on a yearly basis although I'm sure it exists. That said, I think we're talking about tenths, hundredths, or thousandths of a degree difference so it's not a radical movement.

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Clarification for the last sentence: according to How precise can we be with latitude and longitude? 1 second of latitude is almost 100 feet, therefore the Circles change about 0.5 seconds per year. This works out to 150m every 10 years, assuming constant rate of change, which it likely isn't. 150m is 3mm on a 1:50,000 hardcopy map, which is 3 times more than enough to send one back to the drawing board when one's cartographic-excellence minded supervisor sees it. –  matt wilkie Oct 18 '10 at 20:55
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