Here are two facts that seem to argue with each other. Over the long haul, Earth is slowing down. The pull of the Moon’s tides has been stretching our days out for hundreds of millions of years, adding about 2.3 milliseconds per century to the length of a day. And yet for the past several years the planet has been doing the opposite, spinning slightly faster, setting one shortest-day record after another. The current mark is 1.66 milliseconds short of a full 24 hours, clocked on 5 July 2024.
Both things are true at once. The slow tidal drag is still there in the background. Riding on top of it are shorter, faster wobbles, and lately those wobbles have been pushing in the speed-up direction. That’s the puzzle, and it’s stranger and matters more than a few thousandths of a second sounds.
How we know a day was 1.66 milliseconds short
A “day” is really two different things. There’s the calendar day of exactly 86,400 seconds, a fixed unit tied to how we define the second with atomic clocks. And there’s the actual time it takes Earth to complete one turn, which drifts around that fixed number by tiny amounts. On average, Earth completes one full rotation in 86,400 seconds, give or take a millisecond. The gap between the two is what scientists call the length-of-day variation.
These wobbles are far too small to have measured before atomic clocks arrived in the mid-twentieth century. A human blink lasts around 100 milliseconds. The record-setting day on 5 July 2024 ran short by less than one-fiftieth of that. You could not feel it, and no ordinary clock would show it. Only an instrument that keeps time by counting the vibrations of atoms can catch the planet running a hair fast against the calendar.
For context, the shortest day on record held steady at 1.05 milliseconds short from the start of atomic-clock records until 2020. Then the records started tumbling. June 2022 set a mark of 1.59 milliseconds, and July 2024 beat it.
What speeds the spin up
Several things nudge the rotation rate, and they work on different timescales. The Moon’s position matters day to day. So does the way mass shifts around the planet with the seasons. As leaves grow and weather patterns move, that mass gets redistributed, and like any spinning object, Earth responds to where its weight sits. Shifts in wind and air circulation account for roughly 90% of the seasonal wobble in day length.
The multi-year speed-up is harder to account for. The seasonal and lunar effects don’t add up to a steady trend, and the surface systems scientists can model don’t seem to explain it. The leading suspect is deep down, in the planet’s molten core. Leonid Zotov, an Earth-rotation specialist at Moscow State University, put it plainly: “Most scientists believe it is something inside the Earth. Ocean and atmospheric models don’t explain this huge acceleration.” As for the cause, he told the same outlet, “Nobody expected this. The cause of this acceleration is not explained.”
However, a few record years shouldn’t be read as a permanent shift. Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography, offered an analogy that keeps the scale right: “It’s like watching the stock market, really. There are long-term trends, and then there are peaks and falls.” A run of fast years is a peak, not necessarily a new direction. Zotov himself has since suggested the acceleration may already be easing.
The twist: melting ice is slowing us back down
While the core has been speeding the planet up, climate change has been working the other way. As polar ice melts in Greenland and Antarctica, water moves from the poles toward the equator, and that shift in mass slows the rotation.
Agnew reached for a familiar image to explain it. “If you have a skater who starts spinning, if she lowers her arms or stretches out her legs, she will slow down,” he said, as Smithsonian magazine reported. Meltwater spreading toward the equator is the planet’s version of the skater’s arms coming down. His 2024 study, published in Nature, found this ice-melt braking has been strong enough to cancel out some of the core-driven speed-up, delaying a looming timekeeping problem by a few years.
Why a few milliseconds becomes a real headache
This is where an imperceptible number reaches into the systems that run modern life. Since 1972, Coordinated Universal Time has been kept in step with Earth’s actual spin by inserting the occasional “leap second.” Because the planet had generally been running slow, every leap second added so far has been positive: a second tacked on to let the clocks wait for the Earth to catch up.
The recent speed-up flips that logic. If the trend holds, timekeepers may for the first time need to remove a second rather than add one, a so-called negative leap second. Agnew warned that such a step “has never happened before, and poses a major challenge to making sure that all parts of the global timing infrastructure show the same time.” Software makes it worse: it’s likely to be tougher to skip a second because most programs are built to add time, not subtract it.
The oddest part is that melting ice bought the world time. Agnew’s modeling suggested the reckoning would have arrived around 2026, but the ice-melt braking pushed it back. Without the effect of melting ice, Earth would need that negative leap second in 2026 instead of 2029, Agnew calculated. A problem created deep in the Earth is being partly held off by climate change at the surface.
How firm is any of this? Not very, and the experts disagree openly. Dennis McCarthy, the retired director of time at the US Naval Observatory, framed it as near-inevitable: “We are headed toward a negative leap second. It’s a matter of when.” Judah Levine, a physicist at the National Institute of Standards and Technology, has pushed back on confident forecasts, noting that “the fights are so serious because the stakes are so small.” Agnew’s own figures put the odds of needing one by 2035 at only around 40%. Possible, not certain.
What lingers isn’t the record itself. A day 1.66 milliseconds short is far below anything a person could ever notice. The interesting thing is where such a tiny number lands: in the machinery of global timekeeping, in a subtraction no one has ever attempted, and in a tug-of-war between the planet’s molten core and its melting ice caps that we can measure precisely and still can’t fully explain.