The Day the Earth Spinned Too Fast

It’s a matter of milliseconds. Not even time to say “phew”. On June 29, the Earth took 1.59 milliseconds less than expected to go around itself. A rotation of less than 24 hours which above all marks a speed record for the planet, according to the specialized site Time and Date.

Days that are not always 24 hours

Depending on the tides, the movements of air masses in the atmosphere, and the movements inside the Earth’s mantle and core, the duration of the rotation of the Earth on itself varies each day. But, for the past two years, the planet has been experiencing mysteriously very short days. On July 19, 2020, the 24 hours had been cut by 1.47 milliseconds. Ditto on July 9, 2021. Unheard of since the beginning of atomic clocks in the 1960s, which make it possible to precisely measure the rotation time of the Earth.

Especially since this movement should on the contrary slow down, according to calculations made more than a hundred years ago and checked regularly. A phenomenon largely due to the Moon which, by its gravitational force, deforms the terrestrial sphere and slows down its movement a little, like a cosmic brake. In 2016, British researchers calculated that Earth days were not losing, but gaining on average 1.8 milliseconds per century.

A boost that throws the GPS out of whack

So why this boost in recent years? Scientists don’t have an explanation yet. Some see it as the effect of convection in the heart of the Earth, others of climate change at work. With the melting of the ice caps and the modification of precipitation, the masses of air and water are distributed differently on Earth, which could indeed play a role. But for the moment, everything remains to be demonstrated and the only sure thing is that these accelerations occur more often in the summer.

In daily life, whether the rotation takes a millisecond more or less does not upset the watches. But this affects, on the other hand, GPS, whose satellites carry high-precision atomic clocks. In the financial sector and telecommunications, a delay of a few milliseconds can also have consequences on money and data transfers. It will therefore be necessary to reprogram the clocks if the trend continues…


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