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Comment by n0n0n4t0r | Hacker News Reader

One could argue that the moon is orbiting the sun. The fact that it's orbit is a little wobbly because of interferes from the earth is a rounding detail, no?

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> An enduring myth about the Moon is that it doesn't rotate. While it's true that the Moon keeps the same face to us, this only happens because the Moon rotates at the same rate as its orbital motion, a special case of tidal locking called synchronous rotation.

https://science.nasa.gov/resource/the-moons-rotation/

My colleagues once spent a good hour trying to explain this fact to me and I still really struggle to accept it. I can see that the moon is rotating on its own axis from the point of view of a space that is external to the system it forms with the earth. But then isn’t everything on earth rotating about its own axis with respect to that external space? It seems arbitrary to isolate the moon from all this other stuff and make a special case of it…

Pick the sun as reference: the moon rotates. Pick the earth as reference: the moon rotates. Stand on the moon and pick any star as reference: the moon rotates.

From which reference frame would it not rotate?

Yes but from the point of view of the earth the moon does not appear to be rotating around its own axis since it is tidally locked. In that sense it’s confusing to me to distinguish it from everything else on earth but the comment above about centripetal force clarifies this for me I think

We can see the stars moving behind the moon, thus it must be rotating with us.

1. Unlike position and velocity, which are relative (there is no given "origin" for them, no way to say where a thing is or how fast it's moving except relative to other things), rotation is absolute. A thing is either rotating or not, regardless of its relation to other things. Objects that rotate "experience (centrifugal) forces as a result" or "require (centripetal) forces to hold them together" depending on how you choose to describe it. This is detectable: hook two weights together with a newton-meter in space and the newton-meter will read non-zero when the assemblage is rotating, zero when not. The reading tells you how fast it is rotating regardless of any external reference point. (An equivalent device to detect position or velocity is not possible, but it is for acceleration.)

2. Yes, everything "at rest" on earth is in fact rotating at the rate the earth rotates. If you stand on the equator at midday and do not rotate you will be standing on your head at midnight.

>no way to say where a thing is except relative to other things

This is always true. The origin is just a thing that other things are relative to. It's just as possible to define an origin in the real world as it is on a piece of graph paper.

Thanks for this explanation. If I understand correctly then, the moon requires some centripetal force in order not to dissipate due to its rotation whereas e.g. my head or the Eiffel Tower do not because they are not subject to absolute rotation.

They're rotating too.

If you rotate as part of some larger rotating thing then you still rotate. (You also move around.) It's all absolute.

Indeed. The Eiffel tower and your head do both have some (extremely small) centripetal force compensating for their rotation along with the earth.

(You can break that down in different ways, i.e. use various choices of generalised coordinates to describe it, so exactly what constitutes "centripetal", "centrifugal", "gravitational", "tidal", etc. forces depends on that. I'm being pretty vague in how I decribe it. Regardless, rotation is absolute, or in other words the equations of physics take a different form in a rotating frame of reference than in a non-rotating one.)

Thanks for the clarification I completely mistook what you were saying. This is the fascinating bit for me then, that what’s happening with the moon’s rotation is also happening with everything else

Why is the Eiffel Tower not rotating?

It is.

> But then isn’t everything on earth rotating about its own axis with respect to that external space?

From the point of view of the moon, for the purposes of action due to gravity, anything on Earth is essentially part of the Earth, not an entity that is massive enough to be considered separately. The aggregate centre of mass is what counts. Similar for the Sun looking at the Earth/Moon system: from that PoV Earth+Moon is it a single mass with a centre somewhere between the two major masses that form it.

If the Moon where sufficiently consistent in its shape and density, it could rotate freely in any direction while orbiting the Earth, that fact that it is more dense on one side means that it is more energy efficient for it to spin in step with its orbit such that the dense side keeps facing us. If something massive hit the moon (let's assume this somehow happens without significantly affecting its orbit or causing significant problems for Earth too!) it might push the rotation off for a bit, but it would slowly be pulled back into sync. If something sufficiently massive simply landed on the moon, that would affect the mass distribution and the exact face that points at us would slowly change to reach a new equilibrium.

But is a geostationary satellite going around the Earth?

No, because it never sees its back!

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