Friday, March 29, 2013

Why Doesn't the Moon Spin?


In 1992, shortly after launch, the Jupiter-bound Galileo probe took photographs of the moon as it flew past.

I've been asked on a number of occasions recently why the moon always seems to show us the same face -- the lunar nearside. I'm not sure why there's the sudden interest, but it's a very good and valid question, especially as tonight (March 27) is a full moon. 

Look at the moon at any time and -- aside from the constantly changing phases that are caused by changing relative positions of the Earth, the moon and sun -- it does indeed show us the same face, constantly. 

Perhaps surprisingly, it's 'non-rotation' (from our perspective) comes from its interaction with the Earth.

Both the Earth and moon are big lumps of rock with the moon in orbit around the Earth or, more precisely, both objects in orbit around their common center of gravity -- known as the 'barycenter.' It just happens that this point lies very close to the center of our planet, so to all intents the moon orbits us.

For the rest of the story:

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