Our home the planet is busy. On June 29, 2022, Earth completed the shortest day since scientists began keeping records in the 1960s, completing a full rotation 1.59 milliseconds faster than usual.
Terrestrial rush is a trend. In 2020, the planet recorded its 28 shortest days on record, and it continued to spin rapidly into 2021 and 2022. Before scientists could even confirm the record-breaking day of June 29, our world almost outdid itself: It blazed to 26 .July, 2022, 1.50 milliseconds ahead of schedule.
We will likely see more record-short days as the Earth continues to accelerate, said Judah Levine, a professor at the University of Colorado-Boulder and a longtime expert at the National Institute of Standards and Technology (NIST). That the Earth’s days are getting shorter is no cause for alarm, he says, because the actual time difference amounts to fractions of a second over the course of a year. But what’s strange is that while scientists know that changes in the Earth’s inner and outer layers, oceans, tides and climate can affect how fast it spins, they don’t know what’s driving the current rush.
Nobody’s perfect – not even our planet. On average, the Earth rotates on its axis every 24 hours, or every 86,400. second. But for various reasons, from the planet’s imperfect shape to its complicated interior, each day is not exactly as long as the day before.
Also, a day lasting exactly 24 hours is just a standard we’ve come to expect right now. The Earth’s rotation slows in the long term thanks to the Moon’s pull on our world. Just a few hundred million years ago, for example, an Earth day was only 22 hours long. For millennia to come, an Earth day will last much longer.
So what’s with the shorter days of late bucking the long-term trend? One hypothesis that has been put forward so far has to do with the “Chandler swing.” The phenomenon was discovered in the 19th century and explains how the not-quite-perfectly-round Earth wobbles a little, like a spinning wheel, as it slows down. Leonid Zotov told timeanddate.com that the wobble had mysteriously disappeared between 2017 and 2020, which could have helped Earth finish the day a little faster.
Another idea is that climate change could affect the rotation rate of the planet. As glaciers melt into the ocean, the Earth’s shape changes slightly, becoming flatter at the poles and bulging at the equator. But Levine says this effect can’t explain why the planet would suddenly spin faster because melting glaciers would have the opposite effect: The planet’s moment of inertia would increase, which would slow us down.
For Levine, the likely culprit is more mundane.
“One of the possibilities is the exchange of momentum between the Earth and the atmosphere,” he says. “The sum of these two is a constant, which means that if the atmosphere slows down, for example, the Earth will speed up. Or conversely, if the atmosphere speeds up, the Earth will slow down.”
The same thing can happen deep inside our world: It is possible for the deep core and the mantle – the large layer that exists between the core and the surface – to move at slightly different speeds. There could be an exchange of angular momentum between the Earth’s deep core and the mantle, he speculates.
“Both of these effects … can either pump velocity into the Earth’s surface, or take velocity out of the Earth’s surface,” Levine says. But the dynamics of the atmosphere and the Earth’s interior are so complex that it is impossible, at least right now, to point to one of these factors as the sure cause of the planet’s rapid pace.
Nature does not always keep up to the rigidity of a clock or a calendar, and planetary timekeepers are used to making a few adjustments. A leap year exists, for example, because we need an extra day every four years to keep the 365-day calendar in sync with the Earth’s revolution around the sun. Because the day gets longer over time as the Earth’s rotation rate slows, timekeepers throw in a leap second every now and then to keep human time in sync with the solar system.
As the Earth accelerates, we face an unprecedented opportunity: Adding a “negative leap second.” In other words, says Levine, if the planet continues to spin too fast, watchmakers may have to delete a whole second by the end of the decade. For example, they can make the clocks jump from 23:59:58 on December 31, 2029 to 00:00:00 on January 1, 2030.
“If you had asked me about the negative [leap second] five years ago,” says Levine, “I would have said, ‘Never.’ But in the last year or two, Earth has definitely gained momentum. And now, if that rate were to continue — and it’s a big one if there – then we might need a negative lead number two in about seven years, maybe eight.”
This has never been done before. Some researchers wonder if that could introduce a troubling hiccup into computer systems. Considering how the world continues to surprise us, Levine is not yet convinced that time will come.
“You have to remember that it requires an extrapolation over six years – and we’ve been burned before about extrapolations. So I wouldn’t be ready to bet on the farm.”