Although you surely couldn’t have felt it, the Earth just experienced the shortest day ever recorded on July 9th, 2025, confirmed by the International Earth Rotation and Reference Systems Service (IERS), responsible for maintaining global time. Thanks to an uptick in the Earth’s rotation speed, anywhere from 1.3 to 1.6 milliseconds was shaved off the average 24-hour day—and it’s set to happen again!
Scientists predict July 22nd and August 5th will join the list among Earth’s shortest days on record. But first, let’s understand how and why the Earth is spinning faster than usual.
The speed in which the Earth rotates, though quite stable, can be a little inconsistent. So although we measure days by a 24-hour scale, the length of an individual day can vary, explains Space.com. The Moon’s gravity plays a major role in gradually slowing down Earth’s rotation through a process known as tidal braking.
However, on July 9th, 2025, the moon was at its farthest point from the Earth’s equator, altering the angle of its gravitational pull on the planet. This shift contributed to a slight increase in the Earth’s rotational speed.
July 22nd and August 5th are expected to experience similar speed increases, as both days the moon will be far from the Earth’s equator, reports BBC Sky at Night Magazine. While it technically only creates a possible difference in milliseconds— undetectable to us—the change is critical for scientists using atomic clocks.
Atomic clocks use atoms’ resonant frequency to track time with intense accuracy, critical in research, telecommunications and so on. So why does any of this matter? These milliseconds can actually impact the accuracy of UTC, which is based on atomic time and kept in sync with Earth’s rotation.
Moreover, when the Earth gradually slows due to the moon’s gravity, scientists sometimes need to add a “leap second,” which is an occasional one-second adjustment to UTC to ensure its within 0.9 seconds in sync with the Earth’s irregular rotation. If the Earth consistently increases its rotation speed, scientists could potentially have to subtract a second from UTC, known as a negative leap second, which has never been done.