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Since the moon is tidally locked with earth, we always see its same face. How then did we realize that the moon may also be rotating about its own axis?

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    $\begingroup$ I don't understand the question, can you expand / explain what you mean? $\endgroup$
    – MikeB
    Commented May 30 at 12:31
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    $\begingroup$ If it wasn't rotating around its own axis, we wouldn't always see the same face. That is kind of the definition of tidal lock. $\endgroup$ Commented May 30 at 13:34
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    $\begingroup$ Probably a similar question would be "How did they realize that the Moon is orbiting the Earth?" Presumably even before the Earth's shape was known it was intuitive that since the Moon disappears in the West and reappears in the East that it is going behind the Earth. But I'm guessing they probably didn't think of it as moving in a circle, but more like crossing behind the curtain backstage. But once it was realized that the Earth is round, they would have soon realized that the Moon is making a circular orbit. And then it would be obvious that the Moon is rotating on its axis. $\endgroup$ Commented May 30 at 15:57
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    $\begingroup$ Such questions should belong to Astronomy.SE $\endgroup$ Commented May 31 at 2:36
  • $\begingroup$ Note the moon is not fully locked en.wikipedia.org/wiki/Libration so careful study or making drawings would show differences pointing to a spherical body only approximately pointing towards earth. $\endgroup$ Commented May 31 at 3:25

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The ancient Greeks understood very well how the moon moved through space.

As described in this article from the Library of Congress, in the 400s BC, Empedocles and Anaxagoras both provided evidence for the Earth being a sphere (largely based on the way the Earth's shadow looks on the Moon during a lunar eclipse), and the moon was well understood to be a sphere long before that (just looking at the progression of the moon's phases, it's obviously light and shadow on a ball).

They may not have had the whole heliocentric solar system figured out yet, but knowing that the moon and Earth are both spheres, and that the moon is moving around the Earth, it's clear that the moon must be rotating to keep one face pointed towards Earth. They wouldn't have used the term 'tidal lock', but they well understood that the rotation was happening.

And I don't want to imply that the Greeks were the first or only people to figure this out. They're simply the earliest people who wrote it down in a way that we still have access to -- we know that at the latest, this was all understood in the 5th century BC. It may well have been understood by pre-Egyptian astrologers or neolithic Chinese farmers or the first humans in North America. But since they didn't leave us any notes, we don't know.

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Scientists have discovered that the moon actually used to spin much faster around its axis and has been slown down due to the earth's gravity. Now this speed will remain constant and hence we call it "tidally locked".

If the moon did not indeed rotate on its own axis, we would see other sides of it as it revolved around the earth. But as we see only the same side, it is hence proven that the moon does indeed rotate in it's axis. Such can be later proved by data through artificial lunar satallites.

I have recieved most of my information from the study conducted below, this may help you aswell.

https://www.amnh.org/learn-teach/curriculum-collections/young-naturalist-awards/afpectus-lunae-does-the-moon-rotate-on-its-axis#:~:text=Model%20Earth%20was%20exposed%20to,does%20rotate%20on%20its%20axis.

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  • $\begingroup$ Hmmm, not sure this actually answers the question... $\endgroup$ Commented May 31 at 16:28

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