Earth might be about to spin out its fastest day on record. And no one knows why.

On July 9, 22, or August 5, our planet might complete the fastest spin on its axis ever recorded, breaking last year’s record, according to Graham Jones, an astrophysicist writing for timeanddate.com—a website dedicated to tracking time and time zones.

According to the website, Earth spins faster when the Moon’s orbit takes it significantly north or south relative to the equator (among other factors). The difference in speed is infinitesimal; scientists employ extremely precise atomic clocks to measure the length of a day (LOD) in milliseconds longer or shorter than 24 hours—or 86,400 seconds.

Since 2020, every year has produced the shortest recorded LOD. In 2020 it was 1.05 ms less than 86,400 s, or -1.05. Last year, July 5th came in at -1.66 ms, which is the current record-holder. This year, researchers expect the shortest day of the year to take place either on July 9, 22, or August 5, given that the Moon will be close to its farthest point from the equator on those dates. It remains to be seen whether 2025’s shortest LOD will break the record once again.

“Nobody expected this,” Leonid Zotov, an expert on Earth rotation from the Moscow Institute of Electronics and Mathematics, told timeanddate.com. “The cause of this acceleration is not explained,” he added. “Most scientists believe it is something inside the Earth. Ocean and atmospheric models don’t explain this huge acceleration.”

In terms of longer-term changes, the Moon has actually been slowing down Earth’s rotational speed for billions of years. Some 4.5 billion years ago, a single day on Earth was somewhere between three and six hours long. Tidal forces caused by the Moon are one of the factors at play, making the oceans swell up at certain points and the Earth to lose momentum.

In short, the Moon continuously absorbs some of Earth’s rotational energy, which in turn expands and speeds up its own orbit around our planet. Because of this, the Moon moves around 1.49 inches (3.78 centimeters) farther away from us every year.

Currently, it takes the Moon around 27 days to orbit the Earth. Someday, the speed of the Moon’s orbital period and Earth’s rotation will synchronize, meaning the Moon will only be visible from half of our planet at all times (i.e. Earth will be tidally locked). But no need to worry just yet. According to Astronomy Magazine, researchers predict this will happen around 50 billion years from now.

That means it likely won’t ever happen, since Earth will probably become uninhabitable long before that—and given the way things are going these days, we might wipe ourselves out long before that even becomes an issue.