Mercury rotates at such a rate that the Sun appears to stand nearly still at perihelion, when the tide is strongest.

Is there a mechanism that tends to adjust the orbital eccentricity to improve the match – minimize the tidal torque (integrated over time), or some such?

If so, might there be other stable combinations of eccentricity and rotation coupling, other than the obvious 1:1?

EDIT: I'll rephrase, since most of you seem to think my question was something elementary like “how does Mercury rotate?” or “what the heck is tidal locking anyway?”. I suppose it's not a bad heuristic to assume that the newcomer is completely ignorant, but I wish that assumption didn't override a literal reading of the question.

Each planet's orbital eccentricity varies over time. Mercury's is now suspiciously near to what it would need to be for the sun to stand exactly still at perihelion.

So it's conceivable that in the dawn of time Mercury orbited at some arbitrary eccentricity and rotated at roughly 3/2, then resonance brought the rotation rate to exactly 3/2, and then something adjusted the eccentricity so that Mercury's long axis – the line of the lumps on which the tidal torque acts – tracks the sun more closely around perihelion. (I wouldn't expect a perfect match at perihelion, because though tide is strongest then it doesn't vanish at other times!)

If that adjustment effect does happen, how does it work?

And if that happened for 3/2, could it also happen for other resonances, say 2/1 or 5/4? For an odd denominator (other than 1) does the effect, whatever it is, behave differently (unless Mercury has a weird shape)?

  • $\begingroup$ Not sure what you are asking here - tidal locking is the norm when one body is much larger than the other and they are close. $\endgroup$
    – Rory Alsop
    Commented Aug 15, 2015 at 11:09
  • $\begingroup$ @RoryAlsop - Despite being very close to the Sun and despite being much less massive than the Sun, Mercury is not tidally locked in the sense of a 1:1 rotation rate and orbital rate. It instead has a 3:2 ratio. A 1:1 tidal locking ratio is not necessarily the norm. This is a good question, not a bad one. $\endgroup$ Commented Aug 15, 2015 at 17:51
  • $\begingroup$ I still find the question unclear, even after the edit $\endgroup$
    – Rory Alsop
    Commented Aug 15, 2015 at 19:53
  • $\begingroup$ The question appears to be in the last sentence. He's asking if there are other stable spin-orbit resonance combinations besides 3:2 and 1:1. I answered as best I could using Wikipedia as a source. $\endgroup$
    – userLTK
    Commented Aug 15, 2015 at 22:11
  • $\begingroup$ More generally, he's asking whether something could have changed the eccentricity to make the resonance fit better. $\endgroup$ Commented Mar 15, 2016 at 5:23

1 Answer 1


Over enough time there is a tendency of all orbiting objects to fall into resonance with the object they're orbiting. 1:1 being is most common.

1:1 usually called Tidal Locking, is quite common. The Moon to the Earth. Pluto and Charon to each other. All four of the Galilean moons are tidally locked to Jupiter, likely others.

Mercury's 3:2 ratio is more unusual but not necessarily uncommon, especially for a planet with an eccentric orbit.

See Here

An eccentric orbit likely helps create a spin-orbit resonance other than 1:1 and resonance other than 3:2 should certainly be possible (the Wiki article implies that) though there are no known examples that I know of.

For a really crazy orbital spin - check this out: http://www.businessinsider.com/pluto-moons-weird-behavior-2015-6

  • $\begingroup$ I have a hard time imagining that anyone could read my original post and infer that I didn't already know everything you've just told me. $\endgroup$ Commented Aug 16, 2015 at 1:05
  • $\begingroup$ I provided a link that suggests what you asked is likely, but, maybe take a look at this one. arxiv.org/pdf/1311.4831.pdf bottom of page 3 they talk about the likelihood of 2:1 spin-orbital ratios in prograde super-earths near a red dwarf sun. I've not made it through all 26 pages, nor can I swear by the validity of the research, but it suggests that an attractor for a 2:1 spin-orbit resonance is likely under the right circumstances. $\endgroup$
    – userLTK
    Commented Aug 16, 2015 at 10:46

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