Sunday, December 17, 2006

Right down Santa Claus lane

Here's a riddle for the kids. A man leaves his house for a walk. He walks a mile due south, a mile due east, and a mile due north, and finds he is back where he started. What is the man's name?

Yes, Virginia, his name is Santa Claus. And his house is at the North Pole.

But don't go looking for him there, Virginia. We enjoy a good riddle here at Science Musings, but we don't deal in myths. Here's the cold fact: Santa Claus doesn't live at the North Pole.

I knew from a young age that there was something fishy about Santa's address. At the age of five or six I discovered that the North Pole is in the middle of the Arctic Ocean. I asked my mother about this and she said that the ocean is frozen. Santa's workshop, she said, is built on the ice.

It sounded reasonable, and a little research in the geography book seemed to confirm her story. The Arctic Ocean is indeed mostly frozen, and the ice is typically ten feet thick. Thick enough to support a workshop and an army of elves.

But, alas, Virginia, it's not that simple. For one thing, the sea ice is drifting all the time, in a direction away from Siberia toward Greenland. Soviet and American scientists sometimes set up research stations on the thicker parts of the ice and go with the flow. A station might drift a thousand miles or more during its lifetime.

If Santa built his workshop on the ice at the North Pole, it wouldn't stay there. It would drift away. Next thing you know, elves, toys, reindeers and sleigh would be floating into the North Atlantic on a rapidly melting ice island.

What's that you say, Virginia? Maybe Santa's workshop is built on the floor of the sea, right smack at the North Pole? An underwater factory that Santa enters and exits by submarine?

Hmmm, a clever idea. But even that doesn't quite work. It turns out that the floor of the Arctic Ocean has a tricky way of moving around with respect to the pole.

The geographical North Pole is defined as the place where the Earth's spin axis intersects the crust. But the body of the Earth wobbles with respect to the rotation axis, something an astronomer named Chandler discovered back in 1891. No one is quite sure what causes the Chandler wobble -- probably a shift of mass in the body of the Earth, or in the oceans, or in the atmosphere. The Earth wobbles like the wheel of a car when a tire gets out of balance.

I'll grant you it's not much of a wobble, Virginia. The Earth's crust wobbles about the pole in a circle about 50 feet in diameter every 14 months. Still, if Santa had a workshop on the floor of the sea, it would wobble too.

Uh oh. If we're going to worry about tiny excursions of Santa's workshop from the pole, then there are a few other things we need to think about, especially since Santa will be around for a long time to come.

For example, there's plate tectonics. The Earth's solid crust is like a broken eggshell. The pieces of the eggshell, or plates, move this way and that at the rate of an inch or two a year. In a million years or so that can add up to a real change in Santa's address.

And recently scientists have begun to suspect that sometimes the Earth gets really out of balance. The tendency is for the heaviest part of the planet to move toward the equator, in response to centrifugal force. Over millions of years, the entire crust and mantle can slip by hundreds or thousands of miles with respect to the axis of rotation.

The amazing thing, Virginia, is that scientists can now pinpoint the position of the North Pole with an accuracy of a few inches, and they do it by bouncing laser beams off of the moon or artificial satellites, or by comparing the difference in arrival times at several radio telescopes of signals from quasars billions of light years away.

If all of these wobbles and shifts in the location of the North Pole are due, say, to redistributions of mass deep inside the Earth, then we have discovered something about the inside of our planet by observing the most faraway things we can see in the universe -- the quasars.

So you see, Virginia, with all this drifting and slipping it's simply not practical for Santa Claus to locate his workshop at the geographic North Pole. Not unless he wants to mount his entire operation on a giant sleigh and continually go moving about on the ice.

But what if Santa takes up residence at the magnetic north pole, rather than the geographic pole?

Then we have even bigger problems.

During my lifetime, the magnetic pole, which is determined by electrical currents deep in the Earth's outer core, has moved hundreds of miles from Prince of Wales Island in northern Canada to a place well out in the Arctic Ocean. By the year 2050, if the present course continues, it will have shifted completely across the Arctic Ocean to the island of Severnaya Zemlya in northern Russia.

But not to worry. There is no need for Santa to take up residence at the unsteady pole. He has found a firmer address.

The real North Pole, Virginia, is in your heart.

Discuss this essay and more over on the Science Musings Blog.