Re: Precision of the moon's synchronous rotation

This was a great question, Bill!

In fact, there's a tendency in science to avoid what's called the
anthropocentric fallacy - that is, to think that there is anything special
about the current place and time that the observer happens to find
themselves in. And sure enough, that tendency applies very well in this case!

In fact, the moon as a physical body is pretty highly deformed from a
symmetric sphere: tidal interaction with Earth during the moon's formative
period, when it was in a softer molten state, and continued deformation
throughout billions have years, have caused the moon to be permanently
"weighted" on one side. This extra weight serves as a "handhold", which the
Earth's gravity continues to use to tidally lock the moon: so a short
answer to your question is that the moon will continue to rotate
synchronously with its orbit around Earth, for probably many millions of years.

Now as you may have heard, this tidal locking has a cost: namely, the moon
is moving away from Earth with each successive orbit - moving away by a few
centimeters each year. This means that some millions or billions of years
from now, the moon's distance from Earth may become so great that the tidal
locking is no longer so strict - and eventually may cease to operate. In
that unimaginably distant time, assuming that the Sun has not ejected its
outer atmosphere and already "cleaned off" the surface of our planet, our
distant descendents may get to see the "Far Side" of the moon. But it will
look much tinier in our sky then than now - maybe not even showing a proper
disk to human eyes: that is assuming there ARE still "human eyes" then. :)


One last note: each 27.3 days, as the moon orbits the center of the
Earth-moon system, we DO get glimpses here on Earth of small pieces of the
moon's exclusive "Far Side" real estate! That's because the moon's rotation
is relatively constant over this time period - but it's orbital motion
around Earth is not constant! Instead, the orbit "speeds up" as the moon
nears Perigee each month, and then "slows down" again as it passes Apogee.

This back-and-forth dance is called "libration", and it lets amateur (and
professional) astronomers see selected parts of the Far Side each month, if
they know exactly where and when to look on the moon's visible face.

Thanks for a great question, Bill!

Lew Gramer