What is the Antarctic Circle?
The Arctic and Antarctic circles (at about 66°33’) indicate the zone furthest from the pole at which there is at least one day when the sun doesn’t fall below the horizon in mid summer – or rise at all at the winter solstice. This, of course is the result of the 23°23’ tilt in the earth’s axis. Their corollaries (??) lie in the tropics. The Tropic of Capricorn and the Tropic of Cancer represent the southernmost and northernmost lines respectively of where the sun will be directly overhead on at least one day each year (see diagram).
Because the earth wobbles on its axis, the position of these circles moves each year. In fact it shifts from about 22° to 24.5° and back again over a period of 41,000 years in what is known as the Milankovitch Cycle. At present the polar circles are heading closer to the poles (and the tropics closer to the equator) and as we are about halfway to the turning point the rate of change is close to the maximum rate of about 14.5 metres per year. Right now the polar circles lie at about 66°33’39” whereas in 1996 they were at 66·°33’37”. They will reach 68° about 10,000 years from now.