Starship Asterisk*

Dr. Skeptic wrote:Mercury's rotation is locked in a 3:2 rotation to Earth. Coincidence?

Chris Peterson wrote:I assume you mean it's in a 3:2 resonance with the Sun, not the Earth. And it's no coincidence; tidal locking need not be 1:1.

Dr. Skeptic wrote:Nope, with the Earth. It's either gravitational or magnetic tidal lock or a very strange coincidence. Wikipedia can explain it better that I can.

Mercury's rotational period is 58.7 days, orbital period 88 days.
Every 176 days it completes 3 rotations and 2 orbits.
3:2 spin-orbit resonance.

http://en.wikipedia.org/wiki/Mercury_(p ... _resonance

Dr. Skeptic wrote:

Chris Peterson wrote:I assume you mean it's in a 3:2 resonance with the Sun, not the Earth. And it's no coincidence; tidal locking need not be 1:1.

Nope, with the Earth. It's either gravitational or magnetic tidal lock or a very strange coincidence. Wikipedia can explain it better...

You're reading something wrong. There's no spin or orbital resonance between the Earth and Mercury, or any kind of tidal locking. Mercury is in a 3:2 spin-orbit resonance with the Sun. That is, Mercury rotates three times (3 days) for each two revolutions around the Sun (1 year).

The orbital periods of the Earth and Mercury are unrelated; they are what they are as a result of their individual distances from the Sun.

Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury’s 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.[12] A sidereal day (the period of rotation) lasts about 58.7 Earth days.

176/3= 58.6666

Remembering back to when the rotation of Mercury was first quantified this (unknown) coincidence was quite the stir.