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The ratio for a planet to be tidally locked has to be 1:1, but the ratio for Mercury is 3:2. How is Mercury tidally locked if the ratio is not 1:1?

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    $\begingroup$ Do you have a source saying Mercury is tidally locked? Mercury has a spin-orbit resonance of 3:2. Tidal locking is the specific case where that spin-orbit resonance is 1:1. By that definition, Mercury is not tidally locked. My guess is, whatever source you may have was being loose with the definition of tidal locking and referring to all bodies with a spin-orbit resonance as being tidally locked. $\endgroup$ – zephyr Mar 7 '17 at 14:38
  • $\begingroup$ @zephyr the Wikipedia page on tial locking mentions Mercury in a rotation-orbit resonance section, and implies that spin-orbit locking occurs by similar mechanisms as tidal locking. Multiple other sources also make this connection. Depending on what definition you use, whether this counts as tidal locking may be fuzzy. $\endgroup$ – Cody Mar 7 '17 at 18:50
  • $\begingroup$ @Cody That page makes the distinction that higher spin-orbit resonances are not tidal locking. They specifically state may end up in a so-called spin-orbit resonance, rather than being tidally locked [...] A well known case is the rotation of Mercury, which is locked to its own orbit around the Sun in a 3:2 resonance. They indicate these higher-order spin-orbit resonances (i.e., higher than 1:1) are separate from being tidally locked. $\endgroup$ – zephyr Mar 7 '17 at 18:54
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The simple answer to your question is that Mercury is not tidally locked. You may have seen old books (before 1965) that said it was tidally locked, because it was once assumed to be so. Alternatively, as zephyr said, your source may have been referring to the 3:2 resonance, but that is also not really the same thing.

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It's not tidally locked like the moon is because it is in a 3:2 resonance with the sun. It rotates three times for every two orbits it makes. So it isn't considered a tidal lock because it means they usually need to be in a 1:1 resonance. I think you were referring to Wikipedia, where it said Mercury was in a tidal lock with the sun. A 3:2 resonance would not be considered regular tidal locking, but elliptical tidal locking. Elliptical tidal locking means when a body is in a stable resonance that is not 1:1, so Mercury wouldn't be the best example of a tidal lock, but it would be a good example of elliptical tidal locking.

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The reason that Mercury is not locked into a 1:1 spin orbit resonance is its eccentric orbit. In fact, the 3:2 resonance is the result of weak tides in conjunction with the orbital eccentricity. This is in fact mentioned on the Wikipedia page.

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