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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: (

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 an Astrophysicist

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