WFW - No Dark Side

Sorry, Pink Floyd fans; there is no such thing as the ‘Dark Side of the Moon’.
There is a side which always faces away from the Earth, but it is, on average just
as light as the side which does face us. In fact, since the side which faces away
from us is never eclipsed by the Earth’s shadow, its average illumination is just
ever-so slightly brighter. This doesn’t amount to much, but if we choose to label
one side or the other as ‘dark’, I’m afraid it would have to be the side which faces
us.

So, why does the same side of the moon always face the Earth? Is it mere
coincidence that the Moon rotates on its axis at exactly the same rate at which it
orbits the Earth? Does it have something to do with the origin of both bodies,
that these two movements would be so precisely coordinated?

Actually, it is a phenomenon known as ‘Tidal Lock’. The same interaction
between heavenly bodies that cause our seas to rise and fall twice a day, also
affects the interior of each. The liquid mantle deep within the Earth also moves
just a bit with the tides … not much, but a bit.

The effect of the Earth on the Moon’s interior is much more profound. If we
presume (as scientists do) that the Moon once rotated on its axis at a rate
independent of its orbit around the Earth, the proximity of the much-larger Earth
so close to the Moon would cause incredible internal tidal dynamics with each
rotation. These dynamics, this friction, had a slowing effect on the Moon’s
rotation, as friction tends to slow any motion. Over time—and there has been

plenty of time (~4.5 billion years)—the friction completely overcame the
rotational inertia, and the Moon’s rotation became synchronized with its orbit.
The Moon is not alone in this tidal lock phenomenon. Mercury, the closest planet
to the Sun always shows the same side to the sun, for the same reason. It is likely
that, given enough time any small celestial object near a much larger object will
exhibit the same behavior.

In fact, the tidal pull on the Earth’s interior, mentioned above, has a slowing
effect on the Earth’s rotation. But because the Earth is so much larger than the
Moon, the effect is much slower, and the changes in rotational velocity of the Earth
are much smaller. It is estimated that an Earth day is about 1.7 milliseconds
longer than it was a century ago. While this seems a very slight rate of slowing,
given enough time—and, as I mentioned, there’s plenty of time—the Earth’s
rotation would eventually also slow to the rate of the Moon’s orbit. My only
caveat to this is that the time required is long, and our sun may become a red
giant, and envelope both the Earth and the Moon before this occurs – making the
whole question moot.