Days are getting longer as the Earth’s rotation suffers tiny alterations over time. But you’re probably not going to notice anything anytime soon — a day gets one extra minute every 6.7 million years, a new study estimates.

Sunrise

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Each “day” is the amount of time it takes for the planet to do a full rotation around its own axis. So any shift in the speed of Earth’s rotation will have an inverse and proportional effect on the length of a day — higher speeds shorten the day, slower rotation means longer days.

And the latter case seems to be true. A British team estimates that the average day has gained 1.8 milliseconds each century over the past 2700 years. The speed they calculated is “significantly less” than previous estimates which settled on a rate of 2.3 ms per century — which would translate to one minute every 5.2 million years. Still, retired Royal Greenwich Observatory astronomer and lead co-author Leslie Morrison admits that it remains “a very slow process.”

“These estimates are approximate, because the geophysical forces operating on the Earth’s rotation will not necessarily be constant over such a long period of time,” he added.

“Intervening Ice Ages etcetera will disrupt these simple extrapolations.”

The previous figure of 2.3 ms was estimated from calculations of the Moon’s effect on “Earth-braking” — its gravitational force pulls on the Earth’s water and land, effectively pulling against the force of rotation.

But Morrison and his team also factored in gravitational theories about the Earth’s movements around the Sun as well as the Moon-Earth interactions, to calculate the timing of solar eclipses over time as seen from our planet. They then calculated where on Earth they’d be visible from, and compared the results to records of eclipses from ancient Babylonians, Chinese, Greeks, Arabs and medieval Europeans.

“We obtained historical, relevant records from historians and translators of ancient texts,” explained Morrison for the AFP.

“For example, the Babylonian tablets, which are written in cuneiform script, are stored at the British Museum and have been decoded by experts there and elsewhere.”

They found discrepancies between the points eclipses should have been observable from and where they were actually seen. This discrepancy can only be caused by a rotational speed different from the one that the team used in their model (the present one.)

“This discrepancy is a measure of how the Earth’s rotation has been varying since 720 BC” when ancient civilisations started keeping eclipse records, they wrote.

Earth’s rotation speed can be influenced by the Moon’s breaking effect, electro-magnetic interaction inside the planet (between the solid core and the mantle that floats over it), as well as mass shifts on the planet — changes in sea level, shrinking polar caps since the last Ice Age, large reservoirs, and so on.

And while most of us probably won’t ever notice this increase, it’s vital that scientists know about it. This information can be used in adjusting high-precision clocks for example, which underpin our navigational systems.

The full paper “Measurement of the Earth’s rotation: 720 BC to AD 2015” has been published in the journal Proceedings of the Royal Society A.

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