Wobble
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As I speak, we are racing around the sun, spinning as we go. Can you feel it? Didn't think so. Then you probably can't feel us wobble either. It's a slow, subtle wobble indeed, yet obvious enough that Greek astronomers noticed it 2,000 years ago. Hi, I'm Dave Thurlow. Welcome to The Weather Notebook.

Imagine Earth as a spinning top, tilting 23.5 degrees away from the vertical. The spin axis has a small wobble, like a top does as it slows down. This is caused by the fact that Earth is not a perfect sphere but bulges a bit around the middle (like me). Presently, the North Pole points toward Polaris, the star we call the North Star. But that is slowly changing as the wobble moves the spin axis away from Polaris. In 7,500 years, it will point a new North Star. When the wobble finishes one conical circuit of the heavens, Polaris will once again became the North Star. That will take 26,000 years.

Astronomers term this wobble the "precession of the equinoxes" because by slowly changing the direction the spin axis points into space, it changes where in Earth's orbit the equinoxes and solstices occur.

How will this affect the climate? Currently in the 26,000-year wobble cycle, the North Pole points away from the sun when the earth is closest to it in its orbit. In 13,000 years summer and winter will flip flopChristmas in July! and seasonal temperature extremes will increase. According to some climatologists, this subtle wobble, that we'll never be able to feel, is partly responsible for the repeated pulse of the ice ages over the past 900,000 years.

Thanks go to contributing writer Keith Heidorn, and the National Science foundation. Our show is a production of the Mount Washington Observatory, mountwashington.org.