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Comment by leeoniya | Hacker News Reader

i was about to say, wouldn't an uneven mass distribution large enough to throw off probe trajectories also cause the moon to rotate off-axis until it readjusts its shape? then i remembered that the moon doesn't rotate, maybe precisely because of this!

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It seems that the moon does rotate: http://curious.astro.cornell.edu/question.php?number=142

At least to an observer sitting in a fixed point in outer space.

Edit: better explanation: http://www.badastronomy.com/bad/misc/moon_spin.html

okay, that's being a bit nitpicky, i meant with respect to Earth. but maybe you're that guy who goes into detail about galactic rotation when someone asks you to stand still for a photo :)

Sorry, it was not meant to sound nitpicky. I was surprised that I actually had never stopped to think about Moon's rotation, and that maybe it did not spin to a stationary observer.

The more you know...

As long as you're onto spinning things, check out Mercury's rotation.

Moon rotating has nothing to do with from where the observer is observing it. An object is rotating if it produces centrifugal force.

Or rather, the other way round...

Circular motion can be thought of as analogous in many ways to linear motion. Consider that you are on the surface of Earth, which is rotating. Therefore you are also rotating, however relative to Earth, it makes sense to say that you are not rotating. Similarly with the moon. If it were rotating relative to Earth, then we would expect to be able to see what is on its current dark side, as it would be the lit side after half a rotation.

Also, a rotating object does not need to produce or receive any force. The conservation of angular momentum says that unless a force acts on a system, it will continue to rotate at whatever angular velocity it is currently rotating at. This is because the system itself is not accelerating. If you look at a particlar subsection of the system (say at an end), then that subsection is accelerating due to a centrifugal force, but that force is contained within the system.

This isn't really accurate. Circular motion is absolute, but linear motion is relative.

Even on the surface of the earth you can tell you are rotating, even inside a completely closed room. For example with a http://en.wikipedia.org/wiki/Foucault_pendulum

The moon does rotate, but the orbit is synchronised such that one rotation occurs per orbit, so from the Earth it appears not to rotate. However, if you photograph the moon over time you find you can see >50% of the surface area (about 60% I think) due to libration. https://en.wikipedia.org/wiki/Libration

Objects have a tendancy to syncronize their rotations. Additionally, what I understand to be, the leading theory on how the moon was made is that it was blown off the earth, in which case conservation of angular momentum should make it start not rotating relative to earth.

"Should make it start not rotating" - what does that sentence even mean?

Sorry, that should read "not start rotating". Essential its Newton's first law, unless there is a force that would make the moon rotate relative to earth, the moon will not do so.

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