All pairs of gravitationally bound bodies tend to "sync" like this.
However, it takes billions of years to become as apparent as the case of the
Moon's sole face toward the Earth. In fact, most Moons in the solar system
are sufficiently small compared to the planets they orbit that, over time,
gravity has proven strong enough to lock the satellites' rotations to match
their orbital periods.
The Moon is tidally locked to the Earth. When two rotating bodies orbit each
other, they raise tides in each other. These tides cause mechanical
friction. So tidal activity absorbs a lot of energy out of the rotational
energy of the bodies. In other words, the energy in the form of rotational
inertia is partially converted into tidal, geophysical changes in the bodies
involved. The Moon's rotational inertia has been
exhausted, converted into geophysical change in the Earth and Moon. The
Moon, being much smaller than the Earth, long ago dissipated enough energy
to lose rotation so that its tidal bulges are now always aligned with the
gravitational pull of the Earth. The Earth still raises a "tide" in the
Moon but it is in a balanced, steady state now and does not stretch the rock
any more -- there's no more spin for the Moon to give up.
Since the Earth is still spinning relative to the Moon, the Earth is
continuously and dynamically being affected by the gravitational pull of the
Moon. The mechanical friction includes fluid flows around the Earth's
surface as ocean tides, motion of the molten core within the Earth as well
as the stretching of solid rock along the planet's crust. (For a sense of
scale, note that maritime tides are recorded in meters while tides in the
solid aspect of the Earth are recorded in centimeters.) This mechanical drag
is a major reason for why the Earth's rotation is slowing. The day length
is slowly lengthening, and has over geological time increased to 24 hours,
up from 18 hours. Eventually, the Earth will no longer spin in relation to
the Moon. Some time many billions of years into the future, the Earth will
turn its same face to the Moon as well.
In comparison, the tidal effect on the Moon is static because the Moon no
longer rotates in relation to the Earth. All these exerted forces are costs
in energy. They have to come from somewhere. The Moon did have a much
higher rotation rate long before anyone was living on the Earth to observe
it, but the tidal forces slowed it down until it reached an equilibrium
point, i.e., where keeping the same face toward the Earth was the point of
least expended energy. Both will still rotate, both keeping the same face
toward the opposite body.
As for why tides occur, the key operative phrase is "gravitational
gradient." This impressive phrase simply means the pull on one side of a
body (the near side) is slightly stronger than the pull on the far side.
Here's a cute example to make the "gravitational gradient" more imaginable.
During a full solar eclipse, the Sun, Earth and Moon align themselves such
that the Moon is exactly between the Sun and the Earth (putting the Earth in
the shadow of the Moon). If you happen to be on the side facing the Moon,
you actually weigh less than if you were on the other side of the planet!
To further underscore the point that there are two tidal bulges, note that
the gravitational gradient produces two high tides, one on the side being
pulled more, and one one the side being pulled less (With low tides on the
sides). This is the reason for why two high/low tidal cycles occur every
day. The Moon also has two tidal bulges. As mentioned above, these are
now static and aligned with the gravitational pull of the Earth.
Contrary to popular belief, however, the more massive side of the Moon does
not point in the direction of the Earth! This might seem like
utter nonsense at first. The Moon's Center of Gravity (CG) is slightly
off-center, and in favor of the side of the Moon facing away from the
Earth. When the Moon finally stopped rotating with respect to the
Earth, the CG was in fact pointing the other way. How is this
possibly a stable state? By itself, it's not. However, the
gravitational gradient and the rotational inertia is more than sufficient to
counteract this slight imbalance caused by the Moon's eccentric,
opposite-facing CG. (Recall that the Moon does rotate in synchronization
with its orbit around the Earth.) It's true that if the CG were toward the
Earth-ward side, the tidal lock would be relatively more stable. The Moon's
tidal lock is very stable, none the less.
"Wouldn't the kinetic energy of an asteroid hit cause the Moon to
rotate with respect to the Earth?" The force of a sudden impact
required to change the Moon's rotational inertia by even 1% would most
likely cause the Moon to break apart. The rotational inertia of the Moon
and the tidal lock with the Earth is sufficiently stable to resist all but a
massive and destructive impact.
At present day, the Earth rotates once in 24 hours while the Moon orbits the
Earth once in about 28 days. At stasis in the distant future, the Moon's
orbital period won't be the present lunar 28 days, but something quite a bit
longer. However, the angular momentum of the system must be conserved. So
as the Moon's pull causes the Earth's rotation to slow, the Moon drifts
farther outward into a longer period orbit. The system will stabilize where
the two bodies are gravitationally locked. The total angular momentum of
the pair will remain constant. (This is not quite accurate. The Sun also
has a tidal effect on both Earth and Moon. This might be better said as,
"The total angular momentum of all bodies involved will remain
"The Moon's orbital period will be longer and will be further out? Is
this backward? Wouldn't the Moon's orbit *speed up* as the Earth's rotation
slows down, and so be closer to the Earth?" No! This might seem
intuitively contrary. Yes, the Earth is pulling the Moon, thereby
transferring energy to the Moon's orbit. However, for a constant level of
The tricky part is this:
- there is more kinetic energy and less potential
energy for a given
mass in a smaller radius orbit (the mass orbits faster), and;
- if a mass moves farther out, kinetic energy is traded for potential
energy (the mass orbits more slowly).
- For a given mass, an orbit with higher energy has a larger radius.
With an elliptical orbit, the ratio of kinetic and potential energy will
vary accordingly. The part that loses most folks is that if given more
energy, the mass will go into a larger orbit, and will orbit more slowly.
Most likely, the confusion results from using a mental model of a rotating
object where the radius of the object is fixed (a steel wheel, for
instance). The radius of an object held in orbit by gravity is perfectly
able to adjust to the gravity of the system. So, by pulling the Moon, energy
in the form of rotational inertia in the Earth is converted into orbital
momentum of the Moon and so the Moon's orbital radius is increased.
"Does the Moon Wobble?" The Moon doesn't really wobble, but we
do see more than 50% of the surface over time. The Moon's orbit is not a
perfect circle, but is actually an ellipse. That means that when it is
closer to the Earth it orbits a little faster; when it is farther out it
orbits a little slower. The rate of rotation of the Moon
itself is constant, though, so when the Moon moves at a different speed in
its orbit, it is effectively changing the rotation rate relative to the
Earth. Sometimes we see a little bit past one edge, and sometimes a little
past the other (this is in the East-West direction). This effect is called
a result we're able to see roughly 50% of the lunar surface over time.
Repeating for emphasis, Libration occurs because the Moon's
a constant angular speed, while the Moon's elliptical orbit has a
varying angular speed. So it's faster when close to Earth, slower when farther
away. Thus, the orbital angular velocity is sometimes slower than the
rotational angular velocity and the Moon seems to rotate a little one way
as seen from Earth. Then the orbital angular velocity increases again,
catches up, and overtakes the rotational velocity, and the Moon seems to
rotate the other way. And so on. Since the orbit of the Moon will never be
circular due to perturbations by the Sun, libration is here to stay.