Our home the planet is in a hurry. On June 29, 2022, Earth completed its shortest day since scientists began keeping records in the 1960s, completing a full rotation 1.59 milliseconds faster than usual.
Earth rush is a trend. In 2020, the planet recorded the 28 shortest days on record and continued to spin rapidly in 2021 and 2022. Before scientists could even confirm this record-breaking day on June 29, our world almost outdid itself: it flared up on July 26, 2022, 1.50 milliseconds ahead of schedule.
We’re likely to see more record-short days as Earth continues to accelerate, says Judah Levin, a professor at the University of Colorado-Boulder and a longtime expert at the National Institute of Standards and Technology (NIST). That Earth’s days are getting shorter isn’t 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 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 speed.
Nobody is perfect – even our planet. On average, the Earth rotates on its axis every 24 hours or every 86,400 seconds. But for a variety of reasons, from the planet’s imperfect shape to its complex interior, each day isn’t exactly the same length as the day before.
What’s more, a day lasting exactly 24 hours is just the standard we’ve come to expect at the moment. The Earth’s rotation slows down in the long run thanks to the Moon’s attraction to our world. Just a few hundred million years ago, for example, the Earth’s day was only 22 hours long. In millennia, an Earth day will last much longer.
So what is it that gives the shorter days of late that are bucking the long-term trend? One hypothesis that has been put forward so far is related to the “Chandler wobble”. Discovered in the 1800s, the phenomenon explains how the less-than-perfectly round Earth wobbles ever so slightly, like a spinning top, as it slows down. Leonid Zotov told timeanddate.com that the oscillation mysteriously disappeared between 2017 and 2020, which could help Earth end the day a little faster.
Another idea is that climate change could affect the planet’s rotation rate. As the glaciers melt into the ocean, the shape of the Earth changes slightly, becoming flatter at the poles and convex at the equator. But Levin says this effect can’t explain why the planet would suddenly spin faster, because melting glaciers should have the opposite effect: the planet’s moment of inertia would increase, slowing us down.
For Levin, the likely culprit is more mundane.
“One possibility is the exchange of momentum between the Earth and the atmosphere,” he says. “The sum of these two is a constant, which means, for example, that if the atmosphere slows down, then the Earth speeds up. Or conversely, if the atmosphere speeds up, then the Earth slows down.
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The same thing can happen deep in our world: It’s possible that the deep core and the mantle—the large layer that exists between the core and the surface—are moving at slightly different speeds. There could be an exchange of angular momentum between Earth’s deep core and mantle, he speculated.
“Both of these effects … can either pump velocity into the Earth’s surface or take velocity away from the Earth’s surface,” Levin says. But the dynamics of the Earth’s atmosphere and interior are so complex that it is impossible, at least at the moment, to point to one of these factors as a sure cause of the planet’s rapid pace.
Nature doesn’t always stick to the firmness of a watch or calendar, and planetary chronometers are used to making a few tweaks. A leap year, for example, exists 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. As the day grows longer over time as the Earth’s rotation rate slows, chronometers occasionally throw in a leap second to keep human time in step 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, then by the end of the decade the clocks may have to erase an entire second. For example, they might cause clocks to 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, the Earth has definitely been speeding up. And now if that acceleration continues — and there’s a big if — then we might need a negative lead for about seven years, maybe eight.”
This has never been done before. Some scientists wonder if this could lead to worrisome hiccups in computer systems. Given the way our world continues to surprise us, however, Levine is not yet convinced that time will come.
“You have to remember that this requires extrapolation over six years – and we’ve been burned for extrapolations before. So I wouldn’t be willing to bet the farm.”
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