Moon Spin
Years ago at a science club meeting, I asked why the Moon didn't rotate and my
question was dismissed with one word: "gravity".
(Albuquerque, New Mexico)
 The
Moon 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.
The moon, 1969, Apollo 12 mission. Photo courtesy of
NASA.
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.
Further Reading:
The Moon by Bill Arnett, California, An excellent site crammed with information and moon
science.
Sun, Moon, Stars, USATODAY.com.
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