Why is the Moon moving away from us?

It’s nothing personal, honestly. Blame gravity…

29th July 2016
Why is the Moon moving away from us? © NASA

Mirrors placed on the Moon allow astronomers to measure its distance from Earth by bouncing lasers off the reflective surface © NASA

The famous English astronomer Edmond Halley first suspected the Moon was receding nearly 300 years ago, after studying records of ancient eclipses. His suspicions were finally confirmed in the 1970s, when laser beams bounced off mirrors put on the Moon by US and Soviet missions showed that it is moving away at the rate of 3.8cm per year.

It’s driven by the effect of the Moon’s gravity on the rotating Earth. Tides raised in the oceans cause drag and thus slow the Earth’s spin-rate. The resulting loss of angular momentum is compensated for by the Moon speeding up, and thus moving further away.

At least, that’s the basic idea, but there’s a problem. At the current rate of recession, the Moon must have separated from the Earth just 1.5 billion years ago – far more recently than geological evidence suggests.

Creationists have used this to question the standard scientific account of the origin of the Earth and Moon. However, astronomers point out that the recession rate will have been slower in the past because of continental drift, which altered the size and depth of the oceans and thus the amount of tidal drag. Taking this into account pushes the date of separation back by several billion years – in line with the geological evidence. 


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