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We never see the dark side of the moon. It rotates so that the same side always faces us.

I heard two (dinner party) theories on this: 1. The moon was made by smashing out a chunk of the earth, and so somehow preserved some orbital rotation from the earth 2. The gravitational drag from the earth to the moon is lowest when the moon is in 'sync' to only show one face to the earth

I don't trust either of these.

My question is: "Why does the face of the moon 'sync' with the earth?"

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  • $\begingroup$ Actually, you can see the dark side of the moon every month during the new moon. ;) $\endgroup$
    – David H
    Commented Nov 26, 2014 at 8:48
  • $\begingroup$ Lunar Libration: apod.nasa.gov/apod/ap010218.html $\endgroup$
    – Wayfaring Stranger
    Commented Nov 26, 2014 at 21:06

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It is called "Tidal locking", or "gravitational locking" or "captured rotation". According the the Wikipedia page on "tidal locking" (check the references for sources), it is due to Earth's gravity causing a small tidal bulge on the Moon, which affects its rotation. Over time, the Moon's rotation, affect by Earth's gravity, makes it's orbital rotation synchronise with its lunar rotation.

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The Earth and Moon have been "together" billions of years and so even realitively weak forces such as gravitation have had a chance to cause profound effects. One of those is the Moon's tidal lock. The Earth too is being driven towards tidal lock - ie at a point in the future only one face of the Earth will face the Moon, but as the Moon's tidal drag on the Earth is relatively lower (compared to the energy contained in the Earth's spin) than Earth's on the Moon, this is not going to happen for very many millions of years.

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  • $\begingroup$ realitively weak forces such as gravitation lol. If you meant "gravity", that's the weakest force on small scales, but the only force on large scales. So it doesn't matter that it's "weak". $\endgroup$
    – Walter
    Commented Nov 28, 2014 at 20:55
  • $\begingroup$ The only force on large scales? Really? Stand outside in the mid summer sun for half an hour then. $\endgroup$
    – adrianmcmenamin
    Commented Nov 29, 2014 at 10:21
  • $\begingroup$ This is not a force, but radiation. Radiation also decays like $r^{-2}$, like gravitational forces (both is simple geometry). $\endgroup$
    – Walter
    Commented Nov 29, 2014 at 17:35
  • $\begingroup$ Gravity is also radiation. $\endgroup$
    – adrianmcmenamin
    Commented Nov 29, 2014 at 21:17

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