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Still Moon

August 30, 2007

A listener asks: Why doesn’t the moon revolve?


Does the moon revolve? I’m Bob Hirshon and this is Science Update.

Listener Tony Skura of Nome, Alaska, called to ask why the moon doesn’t spin. Well, astronomer David Helfand of Columbia University says this is a common misconception. Although we always see the same side of the moon, it is spinning on its axis. In fact, it must be spinning at just the right speed in order to keep the same side pointing at Earth.

DAVID HELFAND (Columbia University):
Because as the moon revolves once around the Earth every 28 days or so, it also rotates exactly once on its axis. So it keeps the same face towards the Earth. This is because of the tidal interaction between the Earth and the moon, which has locked it in this position.

And if your mind is spinning with a science question, call us at 1-800-why-isit. If we use it, you’ll win a Science Update mug. I’m Bob Hirshon, for AAAS, the science society.