Today will be one of the shortest days of the year, all because Earth's spin is inexplicably speeding up.
While the hours of daylight certainly last longer in the summer, the full day of 9 July 2025 will be 1.3 milliseconds shorter than average.
It takes our planet 24 hours, or 86,400 seconds, to make one full rotation around its axis, though this rate does fluctuate by a tiny amount. To track these changes, the International Earth Rotation and Reference Systems Service (IERS) continuously measures the length of the day to a high level of accuracy.
In 2020, the IERS noticed our planet was speeding up, and has been getting steadily faster ever since.
Their data predicts this year’s shortest days are set to fall on 9 July, 22 July and 5 August, when the Moon is furthest from the equator.
The Moon has always subtly affected our planet’s spin through tidal braking, where the Moon’s gravitational pull causes our planet to bulge.
As well as creating the tides, this deformation slowly leaches away momentum from Earth’s rotation, causing our planet to slow down by around 2 milliseconds per century.
This means that for the dinosaurs of the Triassic Era 200 million years ago, a day was just under 23 hours long. Meanwhile, in another 200 million years, the day will have extended to 25 hours long.
The IERS will occasionally add a leap second to the year to make sure high-precision clocks run on time. The most recent leap second was added on December 31, 2016.
When the Moon is further from the equator, the breaking effect isn’t as strong and so these days are a tiny bit longer. However, the times being seen in recent years are a full half millisecond shorter than those seen prior to 2020.
While some events have been known to change Earth’s rotation – the 9.0 Japan earthquake in 2011 shortened the day by 1.8 microseconds – no one knows what’s causing the current trend.
The slowing won’t have any catastrophic effect on our planet – it’s far too short for anyone to notice – it did result in the IERS choosing to skip a leap second in 2025, and may have to take one back in 2029.
Whatever the cause, this is unlikely to be an ongoing effect, and our planet will eventually return to its long-term pattern of winding down.
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