On July 9, 2025, Earth spun a little faster than usual, enough to make the day about 1.3 to 1.6 milliseconds shorter than the standard 24 hours.
That may not sound like much, but it was the shortest day since modern records began.
It’s not a one-off either. Scientists expect two more short days this summer, on July 22 and Aug. 5, all thanks, in part, to the moon.
What makes Earth spin faster?
Earth doesn’t rotate at a perfectly constant speed. While we define a day as 24 hours, in reality, the length of a day can vary slightly from one day to the next due to both internal and external forces acting on the planet.
Over long timescales, Earth’s rotation is actually gradually slowing down, largely because of tidal friction from the moon. The moon’s gravity pulls on Earth’s oceans, creating tidal bulges that act like a brake. This adds about 2 milliseconds to the length of a day every century.
But over shorter timescales, days to months, Earth’s spin can actually speed up and that’s what happened on July 9, and will also happen on July 22 and Aug. 5.
The moon’s role
On July 9, the moon was at its maximum declination, meaning it was positioned farthest from Earth’s equator. This creates an off-center gravitational pull that slightly changes Earth’s axial wobble, leading to a small but measurable increase in rotational speed.
That unusual lunar alignment is the primary cause of the shortened day. Two more high-declination alignments — on July 22 and Aug. 5 — are expected to create similar effects according to the BBC Sky at Night Magazine.
How do we know?
Scientists have been using atomic clocks to monitor Earth’s rotation with millisecond precision since the 1960s, with globally coordinated timekeeping started in 1972.
Atomic clocks can detect fluctuations of just a few milliseconds in the length of a day. By comparing Earth-based time known as Universal Time 1 with International Atomic Time (TAI) scientists can track exactly how much the planet’s rotation varies.
According to the International Earth Rotation and Reference Systems Service (IERS), July 9, 2025, was the shortest day ever recorded using these modern methods.
Could we lose a second?
Yes, and it would be a historic first.
When Earth’s rotation slows down over time, scientists add a “leap second” to Coordinated Universal Time (UTC) to keep civil time aligned with Earth’s actual spin. This usually happens every few years. The last time it occurred was in 2016.
Leap seconds work like this: if Earth falls more than 0.9 seconds out of sync with TAI, the IERS steps in to add a second to the clock, typically on June 30 or Dec. 31.
But now, with Earth spinning faster, we’re facing the opposite problem. If this trend continues, we could soon be ahead of atomic time, which would require removing a second instead.
This would be called a negative leap second, and it’s never been done before.
Some scientists predict that if Earth’s rotation continues to speed up by just a few more milliseconds each year, a negative leap second might be needed around 2029, according to BBC Sky at Night Magazine, though the exact timing depends on future measurements.
