Science·Tuesday·No. 12,481
The Chronicle
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The Moon Is Drifting Away
It retreats a few centimetres every year, and our days grow fractionally longer because of it
The Moon does not hang above us. It falls — continuously, and forever missing the Earth. That endless fall is what an orbit is, and it carries the Moon around us once every 27.3 days at an average distance of about 384,400 kilometres.
The pull works both ways. Lunar gravity draws the oceans into two slow bulges, one beneath the Moon and one directly opposite, and the Earth turns underneath them. That is the tide, arriving twice a day on most coasts.
Because the Earth spins faster than the Moon travels, friction drags those bulges slightly ahead of it. The water pulls the Moon forward; the Moon pulls back. Our planet loses a little spin, and the Moon climbs into a wider orbit.
The near side, photographed at first quarter, when the shadow line throws the craters into relief.
The retreat is about 3.8 centimetres a year, measured by bouncing lasers off reflectors the Apollo crews left behind. Slow, but not nothing: it has been going on for four billion years.
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The same braking long ago locked the Moon’s rotation to its orbit, which is why one face is always turned toward us and the other was unseen until 1959.
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