posted 12-12-2001 09:45 AM            

I'm not that familiar with the various ancient calendars, but I doubt that there will be any way we can predict changes in the Earth's spin.

As almost everyone's aware, the Earth, in addition to traveling around the sun every 365.25 days (which gives us a year), also spins on its own axis every 24 hours (which gives us a day). It's that spinning motion that keeps the earth stable, like a rapidly spinning top is stable enough to stand upright despite the pull of gravity.

Once something starts spinning, though, it's awfully hard to make it change. If you hold a bicycle wheel by the axle in your hands and turn it sideways, it's easy. But if you have someone start spinning the tire and THEN try to turn it sideways, it requires a lot more force.

And the Earth, just like a top, wobbles and is slowing down, because the forces that started it spinning billions of years ago are no longer being imposed. And as more time passes, the wobble will get more pronounced -- just like a spinning top; and sooner or later it will stop spinning - just like the top.

But the earth is so much larger than a top, has so much more energy stored in it, and is operating against relatively less friction (spinning in space rather than on a tabletop) that the wobble and rate of slowing is very, very slow. I'm not sure of the exact number, but the spin is slowing down at a rate of about one second every four or five hundred years. And the wobble is very slow, too, although we can measure it pretty well.

We do this by seeing where the axis points. Right now it points to Polaris, the North Star. This is why that no matter where you are (as long as you're north of the equator) Polaris does not seem to move, and all the other stars appear to rotate around it. Five thousand years ago, the axis pointed to Vega, and will point there again in several thousand years as the wobble causes the Earth's axis to shift.

So the bottom line is that the Earth's axis is changing, but at a rate we can't even tell unless we have very sophisticated instruments. Something did change the Earth's axis dramatically about 4 billion years ago, when another planet the size of Mars collided with the Earth and knocked chunks away -- which formed into the Moon. But that's the only thing I can think of that would change the Earth's axis suddenly.

And if it did, no one would care about where the sun came up the next day!

regards,