The Arctic Circle. Where exactly is it?

Yesterday I took a tour by bus from Fairbanks to the Arctic Circle. It's about 200 miles away by road, most of it on gravel on the Dalton Highway, a 414 mile long road that goes up to Deadhorse, Alaska, paralleling the Alaska Pipeline. Originally I was planning to drive myself, but after reading up on it a bit it looked like I would avoid a lot of risk by going up with a tour group. I'm glad I did. The weather was nasty, there were a zillion trucks, and I got piles of local lore from the tour guide, Mike. We lucked out getting him. He's a local graduate student getting his PhD in Comparative Philosophy who works part time as a guide. He is a heck of a nice guy, as well as being extremely well-spoken and knowledgeable. We got to hear a lot of stories, given that we left around 6:30 AM and didn't get back to Fairbanks until after 11:00 PM.

To make the trip memorable, Mike made a big fuss about denoting exactly where the Arctic Circle was. After we arrived he rolled out a red carpet in front of the sign so that the second "A" in "ALASKA" was centered on a dashed line on the carpet. He said that was the exact location of the "average" Arctic Circle, given that the North Pole wanders about a bit, which affects where the Arctic Circle passes. We all had pictures taken of us crossing into the arctic.

I chuckled to myself at the unrealistic accuracy of the little ceremony, but then on the way home I wondered just how precisely you could know where the Arctic Circle is. Well, the exact position turns out to depend on a number of factors:

• The position of the North Pole. It wanders around a few meters on the timescale of a year. Its future motions cannot be precisely predicted, since they are mostly due to variations in ocean temperature, salinity, and currents.
• Changes in the obliquity of the ecliptic, the tilt of the earth with respect to the Earth-Sun plane. That changes in a 41,000 year cycle in a complex manner. Currently it is changing in such a way that the Arctic Circle is moving about 15 meters per year north. So the sign is very unlikely to be in the right spot now!
• The tilt is also affected by nutation, which is the combined effect of the planets and Sun (mostly the Moon and Sun) on the bulge of the Earth. It can affect the position of the Arctic Circle up to 300 meters or so, depending where we are in another complex cycle of around 19 years.

These factors can be predicted to a precision of a few meters, and measured to a precision of a few centimeters. After looking into this, I'd be surprised if the second "A" of ALASKA in that sign is within even 50 meters of the "true" circle. I guess Mike suspected something along those lines. because on the way out we drove north about a km or so before heading back south to Fairbanks. Good thing we did.