Earth's Rotation Will Speed Up Tomorrow, Set To Make The Day 1.34 Milliseconds Shorter

Tomorrow, on July 22, the length of the day is expected to be 1.34 milliseconds shorter than usual. Though there are clear reasons for the increased rotational speed this summer, the cause of the recent increase in speed is a little puzzling to scientists.

The Earth's rotation has altered significantly over time. Right now, the Earth rotates just over 365 times on its axis in the time it takes to orbit around the Sun – this is our number of days in a year. However, the length of the day has varied in Earth's history, according to various calculations, meaning the number of days it took Earth to go around the Sun in the past has ranged from around 490 to 372 days.

There are all sorts of factors that affect the speed of rotation, such as changing sea levels and shifts within the Earth, though the biggest factor is that the Moon is moving away from the Earth (who can blame it) and as the two bodies interact, the result is the Earth slowing down at a rate of about 1.8 milliseconds per century.

In recent years, we have kept a precise track of the Earth's day length using atomic clocks. Normally, a leap second may be introduced every now and then to account for the slowing of the Earth. This is vital, for instance, to keep GPS operational. However, since 2020, the reverse has been true: the Earth's rotation has been speeding up again. 

In 2020, the 28 shortest days since 1960 were recorded. Every year after that, the record for the shortest day has been broken, with the shortest day on record so far – set in 2024 – being 1.66 milliseconds short of the usual 86,400-second day.

This year in July and August, we are predicted to have some shorter days again. According to preliminary data from TimeAndDate.com, July 9 was approximately 1.23 milliseconds shorter than usual, while July 10 was 1.36 milliseconds shorter.

Looking ahead, July 22 is predicted to be 1.34 milliseconds shorter, while August 5 is set to be 1.25 milliseconds shorter than a usual day.

Although the Moon is thought to be responsible for the long-term slowing of the Earth's rotation, it can also be a cause of acceleration. The closer to the Earth's equator the Moon is, the more drag it has on the Earth. These days are predicted to be the shortest days of the year as the Moon is at its maximum distance away from the Earth's equator.

These shorter days are predictable by astronomers, but the trend is a little unexpected. Since 1972, there have been 27 leap seconds added to account for the decreased rotation rate of the Earth. But since 2016, not one leap second has been necessary, and the IERS has confirmed no leap second will be added this year in June. Nobody is entirely sure why the Earth's trend of slower rotation appears to have reversed in recent years.

“This lack of the need for leap seconds was not predicted,” Judah Levine, a physicist in the time and frequency division of the National Institute of Standards and Technology, told Discover Magazine in 2021. “The assumption was, in fact, that Earth would continue to slow down and leap seconds would continue to be needed. And so this effect, this result, is very surprising.”

“Nobody expected this,” Leonid Zotov, expert on Earth rotation at Moscow State University, told Timeanddate.com. “The cause of this acceleration is not explained.”

"Most scientists believe it is something inside the Earth," he added. "Ocean and atmospheric models don’t explain this huge acceleration."

Other factors that can affect the Earth's rotation include earthquakes. In March 2011, a magnitude 9.0 earthquake struck off the east coast of Japan, shifted the Earth's axis, and shortened days on Earth. 

The quake – the most powerful on record to hit the country – shifted the planet's axis by about 17 centimeters (6.5 inches), and may have moved the main island by about 2.4 meters (8 feet). Like other similarly large earthquakes, it also changed the rotation speed of the Earth.

"Earthquakes can change the Earth's rotation by rearranging the Earth's mass. This is what a spinning ice skater does to make herself spin faster. She moves her arms closer to her body, she's moving her mass closer to the axis about which she's rotating,"  Dr Richard Gross of NASA's Jet Propulsion Laboratory explained to Popular Mechanics in 2011. "And earthquakes do the same thing."

"This earthquake must've moved the mass on average a bit closer to the Earth's rotation axis to make the Earth rotate faster and the length of the day a bit smaller."

By looking at models of the Earth's mass distribution before the earthquake and using estimates of how the fault slipped during the earthquake, Gross was able to figure out how the mass distribution changed. "Then, by conservation of angular momentum, if I know how the Earth's mass was rearranged, then I know how the Earth's rotation must've changed," he said.

All in all, the earthquake sped up the rotation of Earth by about 1.8 microseconds (1.8 millionths of a second). The Indonesian earthquake in 2004, for comparison, sped up the Earth's day by an estimated 2.68 microseconds.

The IERS will continue to monitor the Earth's rotation, as always, and will be able to confirm just how short these days in July and August are and whether we have any new records on our hands.

An earlier version of this story was published on July 8, 2025.