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The Question
(Submitted May 19, 1998)
Is there a scientific reason why the Earth's moon has the
same rotation and orbital periods? (As in, why does the
same face of the moon always faces us Earthlings?).
Also, is the moon the only object in the solar system that
does this sort of thing?
The Answer
The Moon is by no means the only object which is phase-locked; a large
number of the satellites of other planets also share this property.
There is a nice compilation of orbital and rotation periods of the solar
system here:
http://www.solarviews.com/eng/data1.htm#orb
so you can see for yourself just how many.
The basic reason for this phenomenon is the tidal force. The tide of the
ocean is well known; less well-known but equally real is the tidal distortion
of the entire planet --- the continents (and everything underneath) are
deformed daily by the tidal force (of the Sun and the Moon), roughly by the
same height as the tide of the ocean. A moon close to a planet is (relatively
speaking) subject to a much greater tidal force than the Earth.
If a moon has two, diametrically opposed, permanent tidal bulges, then
it's dynamically most stable if one is always pointed towards its parent
planet.
This explanation is incomplete in that there is a lot of geophysics
(structures of the solar system bodies) involved, something I don't
know a whole lot on, but I think it gives you a broad overview.
Best wishes,
Koji Mukai
for Ask a High-Energy Astronomer
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