Why do observers on earth only see one side of the moon?

On Wed, 16 Feb 2000 you wrote:

> Why do observers on earth only see one side of the moon?

We only see one face of the moon because the moon rotates at just the
right speed that it keeps one side facing toward us. Imagine that
somebody is walking in circles around you. If they always face the same
direction, then as they circle you, sometimes you will see their back,
sometimes their face. But if they turn at just the right speed, you will
only ever see their face - never their back.

It is not a coincidence that the moon keeps one face toward the Earth at
all times. This phenomenon is called "tidal locking", and it is seen in
the moons of other planets, as well as in the orbit of Mercury around the
sun! (Although, it is more complicated for Mercury than what I just
explained.)

Thanks for writing!

Sincerely,
Kurtis Williams

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