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. 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 confirmed her story. The Arctic Ocean is indeed mostly frozen, and the ice at the North Pole is typically ten feet thick (or so said the book at the time). 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.

And we won't even talk about plate tectonics.

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.

And now! With global warming!

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

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

The ice is drifting an so is the pole.

There is another approach to the extraterrestrial hypothesis of UFO origins. This assessment depends on a large number of factors about which we know little, and a few about which we know literally nothing. I want to make some crude numerical estimate of the probability that we are frequently visited by extraterrestrial beings.

Now, there is a range of hypotheses that can be examined in such a way. Let me give a simple example: Consider the Santa Claus hypothesis, which maintains that, in a period of eight hours or so on December 24-25 of each year, an out-sized elf visits one hundred million homes in the United States. This is an interesting and widely discussed hypothesis. Some strong emotions ride on it, and it is argued that at least it does no harm.

We can do some calculations. Suppose that the elf in question spends one second per house. This isn't quite the usual picture – "Ho, Ho, Ho," and so on – but imagine that he is terribly efficient and very speedy; that would explain why nobody ever sees him very much – only one second per house, after all. With a hundred million houses he has to spend three years just filling stockings. I have assumed he spends no time at all in going from house to house. Even with relativistic reindeer, the time spent in a hundred million houses is three years and not eight hours. This is an example of hypothesis-testing independent of reindeer propulsion mechanisms or debates on the origins of elves. We examine the hypothesis itself, making very straightforward assumptions, and derive a result inconsistent with the hypothesis by many orders of magnitude. We would then suggest that the hypothesis is untenable.

At one house a second, it would take Santa three years to visit all the houses in America alone.