The Earth-Moon Spin-Orbit Resonance

-- | October 24, 2013

Question: For being a chunk of Earth’s primordial matter the Moon is quite an amazing stellar feat…it orbits synchronously about the earth with an axis spin velocity 1/100 that of Earth and counterclockwise orbit equal in period as that of Earth making it the only orbiting moon that always shows the same face to the Earth . . . due to supposedly chaotic celestial forces . . . how was that possible . . .after countless space achievement man has been able to emulate that millions of years after the fact. . .but they eventually decay and crash back into the planet . . .talk about amazing . . .that’s downright incredible . . .or is it ? . . . — Ramon

Answer: The fact that the Moon and the Earth always show the same face to each other is one example of a “gravitational tidal lock”. Orbits of objects about other objects tend to seek-out a lowest energy state. One such type of low-energy state is for the orbit and rotation of a moon and planet to be in “resonance”. The Earth-Moon system is a 1:1 resonance. The planet Mercury is in a 3:2 spin-orbit resonance with the Sun. This is a well understood phenomenon, and really just a by-product of Newton’s laws of gravity.

Jeff Mangum

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