You may not have noticed it but that the earth's rotation is slowing down and each day last just a little bit longer than it used too. Not too worried, the millennium will arrive on time this evening. But in other years, New Year's Eve might be a little tardy.

I'm Jim Metzner and this is the Pulse of the Planet, presented by DuPont. Owen Gingrich is senior astronomer at the Smithsonian Astrophysical Observatory.

The way the snow piles up in the polar regions essentially changes the shape and balance of the earth. Therefore, the earth can slow down or speed up, depending on meteorological phenomena. In addition, the moon is slowly dragging the earth to rotate more and more slowly.

According to astronomers, the length of a modern day is a little longer than it was a century ago. Enough to warrant the insertion of an extra second into the calendar every year or so, typically on New Year's Eve. Now, while that may not sound like a lot, over the broad scam of time those extra seconds add up.

We can see that because the earth is slowing down now, obviously a day was much shorter in times past. You go back to the beginning of the Cambrian period when complex life first appeared on earth and you'll find the day was about 18 hours long.

And so fine tuning our calendars is a way of keeping pace with the ever changing pulse of the planet.

You can start making calendar, and as you become more and more sophisticated, you realize that there are these little glitches as far as the calendar is concerned. But those aren't irregularities in the universe, that's just how the universe is, and it's a wonderful challenge to keep working toward ever greater accuracy in describing the natural world around us.

Plus of the Planet is presented by DuPont bringing you the miracles of science, with additional support provided by the National Science Foundation.

I'm Jim Metzner