Earth is expected to rotate faster on July 22, 2025, resulting in the second-shortest day ever recorded. Though the difference will only be 1.34 milliseconds less than the usual 24 hours, this minor blip in time has scientists closely watching a curious and unexplained trend.
This rotational quirk isn’t something we’ll feel, but it could have long-term implications for timekeeping. If Earth continues spinning faster, timekeepers may need to subtract a second from atomic clocks by 2029—a move that’s never been done before and would introduce what’s called a “negative leap second.”
Historically, Earth’s rotation has been gradually slowing due to tidal friction from the moon, which drifts farther away every year and saps Earth’s rotational energy. In fact, billions of years ago, a full day on Earth lasted only about 19 hours. For most of modern history, Earth’s rotation was relatively stable. But starting in 2020, it began setting speed records.
The current record-holder is July 5, 2024, when Earth completed its rotation 1.66 milliseconds faster than usual. This year, July 10 took the lead for 2025 so far, spinning 1.36 milliseconds early, with July 22 expected to fall just behind at 1.34 milliseconds. Another contender, August 5, is projected to be 1.25 milliseconds shorter than 24 hours.
Why is Earth suddenly spinning faster? Scientists aren’t completely sure. Some theories point to the redistribution of Earth’s mass due to melting polar ice and rising sea levels, but that may actually be slowing the acceleration. The more likely suspect lies deep beneath our feet: recent studies suggest that a slowdown in Earth’s liquid core may be redistributing angular momentum, causing the crust and mantle to spin slightly faster.
Leonid Zotov, a leading researcher at Moscow State University, says the acceleration is still not well understood. “Most scientists believe it is something inside the Earth,” he told Timeanddate.com. Atmospheric and oceanic changes don’t account for the sudden shift.
Zotov also believes that Earth’s rapid rotation could be short-lived and that we may soon return to the long-term trend of slowing down and lengthening days. Until then, July 22 will spin its way into the history books—if only by a fraction of a millisecond.