The length of the day on August 5 was one of the shortest on record, due to a recent increase in the speed that the Earth spins at. While nobody will actually notice the 1.33 milliseconds shaved off of what everyone would just assume to be another Tuesday, it is just one of a handful of record-short days caused by the planet’s increased spin in recent months, a phenomenon that scientists are at a loss to explain.

While the standard length of a 24-hour (or 86,400-second) day is governed by the time it takes for the Earth to revolve in relation to the Sun, a minute variation in the rate of the planet’s normal rotation is normal, and is caused by a host of factors, such as the gravitational influence of the Sun and Moon, or variations in the rotation of the planet’s interior; tectonic shifts, along with changes in weather and ocean circulation patterns can also affect how fast the world turns.

One major factor expected to have an effect on the Earth’s rate of rotation is the movement of water from the polar ice caps as they melt from the effects of global warming, with the free-flowing mass of the meltwater evening out toward the equator. However, this re-distribution of mass is expected to have a slowing effect on the planet’s spin as it shifts away from the planetary axis, much like how the rate of spin of a figure skater will slow as they extend their arms outward.

But lately, the Earth’s rotation has been speeding up: so far this year, July 9, July 22 and August 5 have fallen short of the 24-hour average by 1.23, 1.36 and 1.33 milliseconds (ms), respectively, deviations that aren’t far behind the largest recorded shortfall of July 5, 2024’s 1.66 ms. Leap-seconds were introduced when detailed measurements of the Earth’s rotation began in 1973, when the cumulative daily variations added up to slightly more than 1.1 seconds over the course of the year. Prior to the day of July 19, 2020 falling 1.47 ms short of normal, the record deviation was only 1.05 ms.

This increase is in contrast to the rate of spin of the planet’s core, which is lagging behind the speed of the surface by about one-fifth of a second, creating the illusion that the core is rotating in the opposite direction; however, there’s no need to worry, everything’s still running counter-clockwise as normal, it only appears to be spinning backward from our point of view.

While a discrepancy on the order of one one-thousandths of a second would go entirely unnoticed by most of population—the blink of an eye lasts ten times longer—many of our large-scale systems would see a cumulative impact that could cause problems. One example would be the impact on GPS systems: even though August 5th’s deviation was only 1.33 milliseconds, that minute change would introduce a navigational error of a little over two feet (62 centimeters) at the equator; the 1.1 second deviation seen over 1973 would result in reported positions being more than a half-kilometer (0.31-mile) out of place.

But so far, scientists are at a loss for why this increase in the rate of spin is happening. “Nobody expected this,” remarked Leonid Zotov, a geophysicist at Moscow State University who is a leading authority on Earth’s rotation. “The cause of this acceleration is not explained.”

“Most scientists believe it is something inside the Earth,” Zotov added, pointing out that even as late as last year, researchers were predicting that the Earth’s rotation would begin to slow this year, instead of the increase in speed that we’re seeing. “Ocean and atmospheric models don’t explain this huge acceleration.”