Here is the challenge. Say that this year you want to celebrate the winter solstice in style; no bonfires in the backyard or comfortable seat at the Arts Centre for you. This year you are going to have a true winter solstice experience and celebrate right at the Arctic Circle.

That might sound straightforward enough. Just drive north on the Dempster Highway until you reach the pull-off with the interpretive signs marking the Arctic Circle; there, at latitude 66°33', you should have the perfect front row seat for the solstice show of total darkness, right?

Well, not really. While the Arctic Circle is usually described as the imaginary line north of which the sun does not rise above the horizon on the winter solstice, that does not mean that it is pitch black there. The sun's rays reflect off of the atmosphere, providing light even when the sun is below the horizon.

And that is not all; there is an even more troubling reality with the Arctic Circle. Unlike the dependable equator, which always circles around the middle of the Earth, the Arctic Circle does not stay put; its location actually changes with time.

The location of the Arctic Circle is determined by the tilt of the earth on its axis, relative to the plane of its orbit around the sun. This tilt, also responsible for the seasons on Earth, measures about 23°27' right now. Subtract that angle from 90 degrees, and you get the latitude for the Arctic Circle.

But the Earth is just a bit unsteady in its orbit around the sun, and the tilt can vary by as much as three degrees. Come back in 5,000 years or so, and the Arctic Circle could run 50-60 nautical miles to the south, putting it somewhere in the northern Ogilvie Mountains.

"The Earth is constantly wobbling in its orbit, either tilting more towards the sun or more away from the sun," explains Chris Burn, a geography professor at Carleton University. "Over 40,000 to 41,000 years, it varies between about 21°34' and 24°36'. At the moment, we are in that portion of the cycle when the Arctic Circle is getting bigger."

But after the tilt reaches one end of its range, it will start heading back the other way, moving the Arctic Circle far to the north of its present location.

How do we know all this? The earth's wobble is one part of what is called the Milankovitch Theory, named after Milutin Milankovitch, a Serbian astrophysicist who worked in the first half of the twentieth century.

He calculated slow changes in the earth's orbit by carefully measuring the position of the stars, and determining the gravitational pull of other planets and stars. According to Milankovitch's models, the shape of the Earth's orbit around the sun also varies, and the Earth's axis changes direction as it rotates.

Milankovitch hypothesized that these cyclic variations have had a major impact on the Earth's climate, even controlling the Ice Ages. No one paid much attention to his ideas until the 1970s, when sediment cores from the ocean bottom showed that the cycles calculated by Milankovitch did indeed correspond to periods of climate change in the past.

Now people like Chris Burns, a permafrost researcher, are interested in the Milankovitch Theory because of its link to climate change, past and future. This is a controversial area of research, and after the New Year, this column will explore some of the climate debates based on this theory.

But here on the eve of the winter solstice, suffice it to say that the location of the Arctic Circle is not one of those constants in life on which one can depend. The good news is that a few thousand years from now, Yukoners will not have to travel as far North to celebrate winter solstice at the Arctic Circle.

Chris Burn can be contacted by e-mail at at crburn@ccs.carleton.ca. or by phone at (613) 520-2600 ext. 3784. More information on the Milankovitch Theory can be found on both the NASA and NOAA web sites.