- Not all days are created equal. Some days are actually a millisecond shorter than other days.
- Since 2020, Earth has notched up unprecedentedly short days midway through the year. It happens again in 2025 around July 9, July 10, July 22 and August 5.
- So why has Earth accelerated? Many factors affect Earth’s spin. But as of now, no one knows for sure.
TimeandDate published this original post on June 16, 2025. Edits by EarthSky.
Earth does not quite spin at a constant rate
Our planet is an almost-but-not-quite-perfect timekeeper. On average, from the point of view of the sun, Earth completes one full rotation on its axis in exactly 86,400 seconds, give or take a millisecond or so.
So, 86,400 seconds is another way of saying 24 hours. A millisecond (ms) is 0.001 seconds. That’s considerably less than a blink of an eye, which lasts around 100 milliseconds.
The only way to measure these tiny day-to-day variations in Earth’s spin speed is with atomic clocks. The first practical atomic clocks began their timekeeping in the 1950s. The number of milliseconds above or below 86,400 seconds is what we call the length of day.
Earth speeds up
Until 2020, the shortest length of day that atomic clocks ever recorded was -1.05 ms. This means Earth completed one rotation with respect to the sun in 1.05 milliseconds less than 86,400 seconds.
Since then, however, Earth has managed to shatter this old record every year by around half a millisecond. The shortest day of all was -1.66 ms on July 5, 2024. Earth should get close to this again in 2025 around July 9, July 10, July 22 and August 5. The newest estimates from July 10 confirm these as the shortest days of 2025. Also, the latest figures suggest the shortest day of the year overall may in fact turn out to be July 10. But this still needs to be confirmed.
Why multiple possible dates?
The orbit of the moon affects the short-term variations in the length of day. Our planet spins more quickly when the moon’s position is far to the north or south of Earth’s equator.
The moon will be around its maximum distance from Earth’s equator on these dates. Input the date into TimeandDate’s Moon Light World Map. This will show you the moon’s position – indicated by the moon symbol – at the time and date you choose.
Why is all this happening?
Why has Earth accelerated, and when will it slow down again? These are difficult questions. Long-term variations in Earth’s spin speed are affected by a long list of factors that includes the complex motion of Earth’s core, oceans and atmosphere. Leonid Zotov, a leading authority on Earth rotation at Moscow State University, said:
Nobody expected this. The cause of this acceleration is not explained.
Most scientists believe it is something inside the Earth. Ocean and atmospheric models don’t explain this huge acceleration.
Early last year, there were indications Earth might be slowing down, and Dr. Zotov predicted that Earth would decelerate. Zotov said at the time:
But the future will show if that’s right.
That prediction turned out to be premature. Yet Dr. Zotov is striking a similar note in 2025:
I think we have reached the minimum. Sooner or later, Earth will decelerate.
Bottom line: The shortest days on Earth for 2025 will be in July and August. But why is Earth spinning faster? It’s a bit of a mystery.
Via TimeandDate
Read more: Why don’t we feel Earth’s spin?