http://www.wonderquest.com/MoonSpin.htm
Q: Years ago at a science club meeting, I asked why the Moon didn't rotate
and my question was dismissed with one word: "gravity".
A: The Moon, of course, rotates--at the same speed as it orbits the Earth.
So, in the 27.32 days it takes the Moon to go around Earth, the Moon also
spins about its axis one full revolution. That's why we always see the same
face of the Moon.

Now, the interesting part: why does the Moon spin about its axis at the same
rate it orbits? In the distant past, the Earth's tidal pull on the Moon
slowed the Moon's rotation to match the time it takes to go around Earth.

This is tricky stuff. You know about the tides on Earth. The same forces
work on the Moon. It isn't obvious there because the Moon lacks water but it
happens. The Earth's gravitational attraction is stronger on the side of the
Moon nearest to Earth and weaker on the opposite side. Since the Moon isn't
perfectly rigid, it stretches out, like a ball of taffy, along the line
between Earth and Moon. If we were on the Moon, we could, theoretically, see
two bulges--one on the side facing Earth and the other directly opposite.

Long ago when the Moon spun much faster, the Moon's tidal bulge preceded the
Earth-Moon line because the Moon couldn't "snap back" its bulges quickly
enough to keep its bulges in line with Earth, says James Hilton, Astronomer,
U.S. Naval Observatory. The rotation swept the bulge beyond the Earth-Moon
line. This out-of-line bulge caused a torque, slowing the Moon spin, like a
wrench tightening a nut. When the Moon's spin slowed enough to match its
orbital rate, then the bulge always faced Earth, the bulge was in line with
Earth, and the torque disappeared. That's why the Moon rotates at the same
rate as it orbits and we always see the same side of the Moon.

In our solar system, almost all moons spin at the same rate as they orbit.
We think the exceptions are ex-asteroids captured so recently that tidal
forces have not yet equalized the orbital and rotational periods.

"Not only did the Earth slow down the Moon's rotation," says Hilton, "but
the Moon is slowing down the rotation rate of the Earth." The Moon, being
small in relation to Earth, has a long ways to go before it slows Earth's
spin rate to the Moon's orbital rate. It will take "twice the age of the
solar system", says Hilton.

One planet, Pluto, is small enough in relation to its moon that it's already
happened. Pluto and its moon both always show the same face to each other

"Greg Neill" wrote in message
news
"Darrell" wrote in message
...
What do you mean the "other's" spin. The moon is rotating, but isn't
spinning.

Sure it is. It's spinning at a rate of once per month.
Otherwise it wouldn't always keep the same face towards
us.