I like to think I have some kind of understanding of the size of the planet, since I've crossed both the Atlantic and Pacific Oceans at a speed comparable to a human running: 12 knots. But what really brings it home is something a bit more prosaic. Every time I cross Florida from Fort Lauderdale to Fort Myers to visit my brother, I take the Interstate highway known as Alligator Alley. It's a straight shot across the Everglades which I traverse at approximately 70 mph. It takes me a little over an hour.

When I get home I always make it a point to pick up my globe (about 12" in diameter) and find Florida, check out the tiny little image, and reflect on the long drive and how much beautiful country there is to see on it. I then try to somehow mentally expand that scaling to the rest of the globe.

I know all the tricks and numbers and mental crutches astronomers have devised to give us some idea of the size of the universe, the "powers of 10" games that go up and down the dimension of size. But it does little to help gauge it all in your mind. There is the dimension of space, the one for time, and I suspect at least one more (complexity); all equally vast. And there may be others.

If the Universe has created us just so that it can understand itself, then I don't think we're ready yet.