Since our Moon is tidally locked with earth, we always see only one side of the Moon. In such a case, how and when did mankind first realize that the Moon is also revolving around its own axis? or for that matter, it is revolving around earth as well? Although we (humans) might have logically deduced that the Moon will have an hidden side (the side behind the visible side), but that need not mean that the moon is revolving. It could simply be that only one side of the Moon is facing earth...- as if it is permanently locked in that position. What made us imagine it could be revolving around itself and around Earth?
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$\begingroup$ Can you please explain a bit the difference to your previous question? astronomy.stackexchange.com/questions/44361/… $\endgroup$– planetmakerCommented Sep 11, 2022 at 14:54
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1$\begingroup$ @Planet maker: Not much of a difference in my earlier question and the fresh one. May be this one is more clearer. Unfortunately I am not convinced/satisfied with the answers - NEITHER THEN, NOR NOW... $\endgroup$– NiranjanCommented Sep 11, 2022 at 17:36
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1$\begingroup$ @James K: Not much of a difference in my earlier question and the fresh one. May be this one is more clearer. Unfortunately I am not convinced/satisfied with the answers - NEITHER THEN, NOR NOW... $\endgroup$– NiranjanCommented Sep 11, 2022 at 17:36
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2$\begingroup$ I think your confusion is locked doesn't mean not rotating. It is permanently locked in position, therefore it is rotating. $\endgroup$– James KCommented Sep 11, 2022 at 17:42
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1 Answer
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Tidally locked means that it's always facing Earth with the same side.
We know it circles Earth because we know there are lunar and solar eclipses.
Thus it cannot circle Earth, be tidally locked AND NOT rotate around its own axis (as it needs to revolve once around its axis in one revolution around earth in order to show the same side to us all the time).
Think of this easy-to-test analogy: Try walking around a tree, facing it all the time. Now consider the directions your face points to when doing so. You cannot face the tree all the time time AND NOT having had your face oriented once North, once West, once East and once South. You thus faced the tree all the time and doing so made one rotation around your own axis while circling the tree.
Thus, if you assume a spherical body and the Moon circling Earth, then you have to assume it rotates around its axis. Even with a geo-centric model of the universe the Moon was assumed to rotate around Earth and this thought and argument can have been made; the Greeks surely knew about this, even if they assumed as Ptolemaios that the Moon circles in or on its Moonsphere around Earth.
answered Sep 11, 2022 at 14:41