Ancient eclipses show how days are getting longer

AS THE well-known Australian philosopher, Kylie Minogue, once pointed out, it can be a source of comfort to remember that, no matter what else is happening, the world still turns. Unfortunately, things are not quite so simple. Thanks to the moon’s gravitational tug, the speed at which Earth spins has been slowing since the satellite’s birth about 4.5bn 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 100m 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. Tracking changes in day length over time is thus of interest. And that requires data. Thanks to the development of super-accurate atomic clocks in the 1950s, and to laser range-finding equipment left on the moon by the Apollo astronauts, researchers have plenty of such data from the past half-century. But more information is always welcome. And extra data are exactly what a team led by Leslie Morrison, a retired professional astronomer, have just provided. In a study just published in the Proceedings of the Royal Society they use observations made by ancient Chinese, Babylonian, Greek and Arab astronomers to reconstruct the history of Earth’s rotation over the past two and a half millennia.

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