On Fri, Aug 26, 2016 at 05:57:23PM +0000, Phil Pugliese (via tml list) wrote:
> This is something of a side issue but, way back when, I remember
> that *everyone*, & every book I read, believed that Mercury was
> tidally locked.  Then, some years later, it was proven that Mercury
> was not, in fact, tidally locked.  I've never read nor heard an
> explanation.  Could it be that it's "on it's way" but just hasn't
> gotten 'there' yet?

Mercury is "temporarily" locked to 3:2 instead of 1:1.

Its orbit is elliptical, and 1:1 tidal locking can't happen until the
orbit becomes close to circular.  The catch is the orbits have
enormously more angular momentum than rotations, so shifting them
takes a lot longer.  If Mercury was the only planet, then eventually
this orbit would shift toward circular and 1:1 locking over many
billions of years.  However, interactions with other planets
(especially Jupiter) are likely to keep Mercury's orbit elliptical.

- Tim