Obviously, the moon orbits the earth. However, does it rotate on its own axis? All the text books (and someone who I emailed in NASA) say that it rotates. I don't think so.

Consider a ball tied to a string, spinning around your head. The motion of the ball mimicks the motion of the moon, one hemisphere always point toward the center of the orbit. Is that ball rotating on its own axis? I don't think so.

Consider a train on a circular track, going round and round. Is that train rotating? Its its motion the same as the moon (one side always facing the center). Would anyone say the train is rotating? I don't think so.

Test it? The Myth Busters could do the ball on the string experiment and film it in slow motion. When the ball is let go, it should go in a tangential direction, and if it doesn't rotate then neither does the moon. If that happens then they would be one up on the whole astronomical community.

The moon always has the same side facing the earth - we always see the same pattern of craters on its surface. On the other hand, as the moon does one orbit around the earth, its phases show that the sun lights up all sides of the moon. In other words, with respect to the sun, the moon has turned once on its axis as it has gone once around the earth.

I would say that in your train example, the train is rotating: It faces North at one moment, South some time later. Sure, it is not in the same spot at that moment, but that does not matter.
If you did donuts with a car, wouldn't you say you were rotating? What if you did very small circles?

Rotation is always defined relative to some fixed reference frame. On the earth surface, you could use compass directions. With the moon, you could use the stars to define the moons rotation more exactly.

What you are doing that confuses the issue, is define rotation of the moon using a frame of reference centred on the earth and pointing to the moon. Unfortunately, the moon orbits around the earth so your frame of reference rotates as well. In that rotating frame of reference, the moon doesn't seem to rotate.

To define rotation properly, you need a non-rotating frame of reference.

Quite right Jaap, I couldn't have explained it better myself (or as well probably), the moon rotates with respect to the sun & the stars, but not with respect to the earth.

Side myth...


We are building wind turbines...
These cause resistance to the wind to generate power...

if we created an abnormal amount of resistance would we affect the rotation of the earth?

Would we cause it to slow (or speed up) its rotation?

Obviously large mountains cause a fair bit of friction as does everything else on the surface, but will adding extra resistance cause a future potential problem? or is everything just spinning around quite happily?