Date: 1999 - 2000
I know that the reason we only see one side of the moon
from Earth is that it does one revolution per orbit. I was surprised to
see that this is quite common in the solar system, yet not always true,
with other satelites orbiting other planets. Has this always been the
relationship between Earth and Moon? And why does it occur?
No. I think newly formed moons always rotate as they orbit. Their rotation
is continuously being slowed down by the tidal force between them and the
planet they're orbiting.
Look at it like this: Forget about the bulk of the moon for a minute and
think of two small rocks, one at the moon's surface nearest earth and one
on the far side. Left alone, those rocks would move in different orbits,
but being attached to the moon, they can't. The near one is not moving
fast enough to be in an orbit that close to the earth; without the moon
holding it, it would fall earthward. The other rock is too far away from
earth to be moving at the speed of the moon as a whole; without the moon's
gravity, it would fly off.
So, the tidal force is continually stretching the rotating moon into a
football pointing earthward, and this produces huge frictional forces.
The moon heats up, and that heat energy came from the moon's rotational
energy, so the moon's rotation slows down.
It's called ``tidal locking,'' and I think several moons of Jupiter
do this, and Mercury is tidally locked with the Sun.
It occurs because while the Moon generates tides in the Earth, the
Earth also generates tides in the Moon (in the solid rock). As long
as the Moon rotates, the tidal bulges move around the Moon as the
tides do on Earth. Twice a lunar day the rock moves up and down, just
as twice an Earth day the tides go up and down. It takes lots of
energy to distort rock this way, and it comes out of the rotational
kinetic energy of the Moon. So, the Moon slows down, and eventually
comes to the point where one side always faces the Earth. Now the
tidal bulge is stationary (facing the Earth) and no more braking
The Earth is being slowed by tidal friction, also. I believe
estimates are that it rotated in 20 hours or so when first formed, so
it has lost 4 hours to the Moon's effect. Smaller bodies have less
rotational kinetic energy, so they become tidally locked faster.
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Update: June 2012