On three days this summer (July 9, July 22, and August 5), the Earth will complete its daily spin a little faster than usual. So fast, in fact, that each day will be about a millisecond and a half shorter.

Most of us won’t feel a thing but in a way, that’s a pretty big difference. Beneath our feet, the planet is picking up speed — and no one really knows why.

A Planet in a Hurry

The Earth has been spinning ever since it was formed. Our solar system started as a vast, swirling cloud of gas and dust, and gravity pulled material together to form the Sun and planets. This rotation movement was conserved, but it is slightly slowing down. A billion or two years ago, a day would have laster around 19 hours. In the Jurassic period, it would have been 23 hours. But in addition to this gradual, slow deceleration, there are also small bursts that are harder to explain.

We now have very good instruments to monitor the duration of the day.

Atomic clocks — among the most precise instruments humans have ever built — have been keeping track of Earth’s spin since the 1950s. But what they show today is unexpected. Our planet, which for decades had been slowing down in its rotation, is suddenly speeding up.

The shortest day ever recorded came on July 5, 2024, when Earth finished its rotation 1.66 milliseconds faster than the standard 86,400 seconds that define a full day. This summer, predictions suggest another day could be 1.51 milliseconds short on August 5, just slightly longer than a blink — but enough to rattle the systems that rely on ultra-precise timing.

Timekeepers are watching closely. They’re not thinking of doing any adjustments — yet. Normally, when Earth slows, scientists add a leap second to keep our clocks in sync with planetary time. But if the speeding trend continues, they may be forced to do the opposite and subtract a second. That’s called a negative leap second, and it’s never happened before.

Why Now?

There are many reasons Earth’s rotation isn’t perfectly steady. Some are expected: the pull of the Moon, the motion of the oceans, the push of seasonal winds. Even the growth of leaves in summer plays a role, as Richard Holme, a geophysicist at the University of Liverpool, explained.

“In Northern summer, the trees get leaves,” he said. “This means that mass is moved from the ground to above the ground — further away from the Earth’s spin axis.” That shift slows the spin, in the same way a figure skater slows down by extending their arms.

But that’s not what’s happening now. Earth isn’t slowing — it’s speeding up.

And the change is enough to puzzle the experts. Several causes are plausible, but none stand out as a clear candidate.

Ocean currents and high-altitude winds can also nudge Earth’s spin. In some cases, powerful earthquakes, like the one that struck Japan in 2011, can shorten a day by a fraction of a microsecond. Glacial melt and groundwater loss, both driven by climate change, also redistribute mass around the globe and subtly affect how the planet spins.

Between 2000 and 2018, NASA researchers found that human-driven mass changes lengthened the day by 1.33 milliseconds per century. That’s the opposite direction of what we’re seeing now.

Not just the Earth

It’s possible the Moon is playing a role. The Moon’s orbit wobbles relative to Earth’s equator. When it swings far North or South of that line — as it will on July 9, July 22, and August 5 — it can temporarily tweak Earth’s spin. That’s why scientists expect those specific dates to be our shortest of the year.

But those effects are well understood. What’s mysterious is the long-term acceleration that began in 2020 and continues today. Earth set record after record that year, including on July 19 and again on July 9, 2021. In 2022 and 2024, more records fell.

2023 was a brief pause. But now, the planet is picking up speed again.

This is more than just a scientific curiosity. Modern life depends on accurate timekeeping. It’s not just about you catching your train or beeing on time for a meeting, in which case a microsecond wouldn’t make much of a difference, but it’s about GPS, communication satellites, financial transactions, and power grids.

These systems depend on Coordinated Universal Time, or UTC, which occasionally adds a leap second to keep up with Earth’s rotation. But if Earth continues spinning faster, a new problem arises: should we remove a second instead?

For now, no one’s rewriting the calendars. The time changes are measured in milliseconds — a thousandth of a second. But time is relentless. Left unchecked, the errors accumulate.

And Earth, it seems, isn’t quite as predictable as we thought.