Earth’s Rotation Is Speeding up This Summer—but Just for 3 Days

Here’s what you’ll learn when you read this story:

Although the Earth completes one full rotation in 86,400 seconds on average, that spin fluctuates by a millisecond or two every day.
Before 2020, the Earth never experienced a day shorter than the average by much more than a millisecond, but in the past five years, it’s been more likely to see days during the summer than are nearly half-a-millisecond shorter than pre-2020s levels.
In 2025, the Earth will continue this trend, and scientists predict that three days—July 9, July 22, and August 5—could be atypically short compared to historical averages.

While many of the astronomical truths of existence feel like immutable facts compared to our relatively puny lifespans, the movement of the heavens is constantly changing and evolving. Take the Earth’s rotation, for example. During the Mesozoic, dinosaurs actually experienced 23 hours days, and as early as the Bronze Age, the average day was 0.47 seconds shorter. 200 million years from now, a standard Earth day will actually be 25 hours long (and it remains to be seen whether or not humans will still complain about there not being enough hours in the day).

While the Earth’s rotation changes over cosmic timescales, it also fluctuates on daily ones. We all know that a day lasts 24 hours, or 86,400 seconds, but that’s not perfectly accurate. Earthquakes, volcanoes, tidal forces, subterranean geology, and many other mechanisms can cause the planet’s rotation to slow down or speed up, and those micro-adjustments can trend over time. Although Earth’s overall rotational trend is to slow down, since 2020, scientists have noticed—thanks to the International Earth Rotation and Reference Systems Service (IERS) at the U.S. Naval Observatory in Washington D.C.—that Earth’s rotation is speeding up. So much so, in fact, that experts expect we’ll need to subtract a leap second for the first time ever in 2029.

A new report from timeanddate.com claims that this fast-rotating trend won’t be slowing down in 2025, either. According to IERS data, the three shortest days (mathematically speaking) this year will be July 9, July 22, and August 5. These are the dates when the Moon will be furthest from the equator, which will impact the speed of Earth’s rotation. Current predictions place the shortest day, August 5, at roughly 1.51 milliseconds shorter than average. That doesn’t quite beat out the recent record holder—July 5, 2024, which clocked in at 1.66 milliseconds shorter than average—but it’s still a full half-millisecond faster than when this rotational trend began in 2020 (and, technically, it could still break the record once scientists measure the actual rotation on the day).

“Nobody expected this,” Leonid Zotov, an Earth rotation expert from Moscow State University, told timeanddate.com. Zotov co-authored a study in 2022 analyzing the cause of Earth’s recent rotational uptick. “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.”

Scientists will continue to study the reasons behind the Earth’s rotational fluctuation, and we’ll all endure at least one leap second skip before abandoning leap seconds completely by 2035. However, Zotov also tells timeanddate.com that this acceleration is not a new trend. In other words, we’re not traveling back toward back toward the Mesozoic in terms of rotation. The planet will eventually continue its steady deceleration—this is, of course, it’s natural tendency, but surface changes like polar ice melt can also contribute to the Earth’s rotation slowing down.

The only constant is change.

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