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Posted by Alexander on March 12, 2001 14:47:21 UTC

Earth and Moon are interacting gravitationally slightly non-centrally because Earth is not spherical - there are 2 tidal bulges on Earth - one due to Moon's pull and the opposite one due to centripetal accelearation of Earth orbiting center of mass of Earth-Moon system.

Due to Earth's rotation they are not exactly under Moon-Earth line, but are slightly ahead (inertia). So near bulge pulls Moon forward about 6.7% stronger than farther bulge pulls it back (becase the farther one is farther away). Thus Moon accelerates forward and gains angular momentum and Earth slows down (so total momentum remains the same).

As a result each Earth's day is 42 nanosecond longer than the previous one and Moon is receeding about 1.5 inches/year away from Earth.

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