So here’s a question you might idly contemplate as you lie back one summer’s evening gazing into the star bright night sky watching as the planets, the moon and the stars lazily move on by, “Will the Earth ever stop spinning ?”. And even if you can’t see yourself ever asking that question … we’re going to try to answer it anyway !
Left to its own devices, the huge mass of the Earth would probably cheerfully spin forever in the frictionless vacuum space. As it is, the Sun and Moon are steadily slowing it down through the tidal drag they exert on the world’s oceans.
The effect is very small: a day lasts barely 0.0017 seconds longer than it did a century ago – negligible on human timescales. However it does add up. For example, when dinosaurs ruled the world, the planet was spinning markedly faster than it does today, making a day around half an hour or so shorter. Studies of the daily growth rings on fossilised coral and molluscs have also shown that a day was just 22 hours long around 450 million years ago.
At the current rate, it will take billions of years for the Earth finally to come to a halt, by which time humans will probably have left for somewhere better. This assumes, of course, that the rate of spin will not change, which is by no means certain.
A Sting in the Tail to Our Answer
So far so good…however if we want to project into the future we need to look at the past as well to see if our theories are right. Herein lies a puzzle.
As the Earth spins ever more slowly, it loses angular momentum to the Moon, which moves slightly further out in its orbit. The predicted increase is very small but it has been detected by bouncing laser beams off equipment placed on the Moon by the Apollo astronauts, revealing that our nearest celestial neighbour is currently receding from us by around 1.5 inches per year. Again, this is negligible on a human scale but it tots up – with perplexing results. Calculating the effect of the Earth’s spin-down on the Moon’s orbit suggests that the two must have been in contact around 2,000 million years ago. Astronomers are fairly confident, however, that the Moon has been a separate entity for at least 4,500 million years. Clearly something has gone wrong with the theory somewhere but quite what is so far, far from clear.