# Who first proposed the theory of tidal locking?

I'm interested in the history of the concept of tidal locking but haven't been able to find any articles presenting a timeline of its development. I'm hoping to have a look at the first published paper on the subject, if possible.

• It's not tidal locking, but the ancient Greeks knew that tides were related to the moon. en.wikipedia.org/wiki/Theory_of_tides. But they didn't know why they were linked. May 20, 2021 at 15:23

The concept of tidal locking appeared, at least qualititively, in proposition 38 of book III of Newton's principia. Newton wrote there:

Hence it is that the same face of the moon always respects the earth; nor can the body of the moon possibly rest in any other position, but would return always by a libratory motion to this situation; but those librations, however, must be exceedingly slow, because of the weakness of the forces which excite them; so that the face of the moon, which should be always obverted to the earth, may, for the reason assigned in Prop. XVII be turned towards the other focus of the moon's orbit, without being immediately drawn back, and converted again towards the earth.

Proposition 38 is in the subsection entitled "forces to move the sea"; this fact shows Newton placed the "tidal locking" effect within the theory of tidal forces.

Here's an upper bound, so to speak, from a paper

This was first “predicted” by Immanuel Kant in 1754, when he proposed that the gravitational force of the Moon would slow down Earth’s rotation until the two objects would be tidally locked.22 While Kant’s argument was merely verbal, it did provide a correct rationale.

Which implies that tidal locking was well-known by his time. That paper seems to state that Newton at least recognized frictional forces due to tides and the implications of this, as they reference his work

3 Sir Isaac Newton,Philosophiæ Naturalis Principia Mathematica (1687).

• I wonder what people thought prior to that. It seems like the fact that we only ever see one face of the moon would have been quite a quandary for early astronomers. May 20, 2021 at 0:01
• @numbynumb: The fact that from Earth, the apparent sizes of the Moon and the Sun are pretty much identical, is down to pure and simple coincidence. There's no reason to not believe that the synchronization between the Moon's rotation around its axis and its orbit around the Earth is coincidentally identical either. May 20, 2021 at 9:59
• It's the only place rotation of a celestial body would be immediately apparent. Without a mechanical explanation, one might be inclined to question the sphericity of the moon. An absolute coincidence of orbital and rotational period is far less likely than two objects having the same angular size (in fact, both the sun and moon's apparent angular size fluctuate and the coincidence is an approximate one). I can't think of another example in nature where two unrelated motions perfectly coincide. May 21, 2021 at 4:18