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Ancient eclipses show how days are getting longer

from The Economist

Thanks to the moon's gravitational tug, the speed at which Earth spins has been slowing since the satellite's birth about 4 1/2 billion years ago. Physicists can calculate from first principles how big the effect should be. It turns out that the moon should be adding about 2.3 milliseconds to the length of the day with each century that passes. This means, for instance, that 100 million years ago, when dinosaurs ruled Earth, a day was nearer 23 than 24 hours. But that 2.3 milliseconds is only an average. Geological events within Earth can speed the process up, or slow it down. more

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