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.
Comment
Readers' Comments:
- If this explanation as to why the moon rotates at the same rate as the
orbit of the Earth is correct, then why does the same not apply to the planets
in relation to the sun and then the Sun in relation to the Galactic core? I
think the conclusion in the Moon Spin page is incorrect.
Rick, Richardson, Texas, USA
- Reply: Actually Mercury does always turn one side to the
Sun for one orbit about the Sun. However, for the next orbit, it turns
its other side to the Sun. It turns exactly one and a half times each
time it goes around the Sun. But the Sun is able to slow Mercury's
speed enough for almost synchronous orbiting.
- The other planets are too far away from the Sun for tidal effects to
accomplish synchronous rotation, yet. Such slowing takes time.
For example, the Moon will eventually slow Earth's rotation so that Earth
always turns one side to the Moon. The Moon, being small in relation
to Earth, has a long ways to go. It will take twice the age of the solar
system.
- When we all line up, i.e. the moon facing the earth, the earth and moon
facing the sun, the sun facing the galactic core etc, (provided our sun
doesn't run outta fuel by then; or even so but on a bigger scale) and all
other solar systems do the same they will be sucked into the center and a new
big bang will occur with the exact same outcome will happen; we will be here
trillions of years later and I will be writing this exact same msg... Think
about it, de ju vue or me just being silly. Don't take things so serious guys!
Trev, St. Andrews, New Brunswick, Canada
- Why is it that we now believe the moon takes 27.32 days to rotate instead of
the previously stated 28.1 as was taught in school in the mid 1980's? Is it a
change in calculation or is it a change in the observation?
With that in mind, what effects does the change (if in fact there was one) have
on time as we measure it? Since we now know that light is not at a constant
rate, does the "pull" of the tidal effect affect our time constant? Does it
affect the rate at which any particle with mass moves?
Shawn, Missouri, USA
- Reply: I'm not sure about the
28.1 days. The answer does differ if you measure by the stars (sidereal
time: 27.32) or the sun (synodic time: 29.53
days). Please click here for more on the two times:
http://cseligman.com/text/sky/rotationvsday.htm
- Does the moon rotate around the earth or the earth rotates around the
moon??? Jessica,
Santa Rosa, Philippines
Reply: They rotate around a common
center of gravity, which is actually inside the surface of Earth.
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