reply to post by Equinox99
If you go out on several different nights and look at the Moon, you will always see the same features, at about the same position. It looks as if the
Moon doesn't rotate! Ah, but it does.
This can be seen using a model. Grab two oranges (or apples, or baseballs, or whatever roughly spherical objects you have handy). Mark one with an
"X"; this represents a feature on the Moon. Now put the other one down on a table; this is the Earth. Place the Moon model on the table about 30
centimeters (one foot) away with the X facing the Earth model. Now move the Moon model as if it were orbiting the Earth, taking care to make sure that
the X faces the Earth model at all times.
Surprise! You'll see that to keep the X facing the Earth model, you have to rotate the Moon model as it goes around the Earth model. Furthermore, you
can see you have to spin it exactly once every orbit to keep the X facing the Earth model. If you don't rotate it, the Moon model will show all of
its "sides" to the Earth model as it goes around.
Now, I have been a bit tricky here. We are talking about two different frames of reference; one on the surface of the Earth looking out at the Moon,
and one outside the Earth-Moon system looking in. You performed the experiment from the latter frame, and saw the Moon rotating. From the former,
however, you can see for yourself the Moon does not rotate. What is being argued here is that in one frame the Moon rotates, in another it does not.
We've actually learned three things:
1) the Moon rotates as it orbits the Earth (as seen by an outside observer);
2) it rotates one time for every orbit (to that observer); and
3) if it didn't rotate, we would eventually see all of the Moon as it orbited the Earth.
There is a bit more to this story. We actually can see a bit more than just the one face of the Moon. Because the Moon's orbit is not a perfect
circle but actually an ellipse, its spin and rotation don't exactly match up. This means that sometimes the spin lags behind the orbital speed, and
sometimes it moves ahead. This in turn means that sometimes we can "peek" around a bit onto the far side of the Moon. This is called "libration".
You can see it yourself! If you happen to observe the Moon a week after perigee (closest point to the Earth) and then two weeks later, a week after
apogee (farthest point from the Earth) you can see that the face looks like it has rotated a bit. This is easiest to see with binoculars or a small
telescope. It's very hard to see with the naked eye, but remember, ancient astronomers knew about this effect long before the invention of the