Earth’s rotation is no longer matching the tidy 24-hour day that our technology assumes. Since 2020, each day has been shorter by roughly 1.5 milliseconds, a tiny sliver of time that is now large enough to matter for the ultra-precise atomic clocks that run navigation systems, financial networks, and the internet. Instead of adding an extra “leap second” to keep up with a slowing planet, timekeepers are preparing for something unprecedented: possibly deleting a second before the decade is out.
That reversal captures a deeper story. For billions of years, Earth has been gradually slowing as the moon’s gravity drags on the oceans, yet in the span of a few decades, internal dynamics and climate change have started to tug the planet in the opposite direction. Our clocks are not just falling behind a spinning world, they are exposing how profoundly humans are now entangled with planetary physics.
How we built a world around a not-quite-24-hour day
Modern timekeeping rests on a compromise between the messy reality of astronomy and the clean regularity of physics. The official global standard, known as UTC, is calculated from an average of over 400 atomic clocks around the world, which tick with astonishing stability. Astronomical time, by contrast, is based on how long Earth actually takes to rotate once relative to the sun, a quantity that drifts as the planet speeds up or slows down. When those two measures diverge by more than 0.9 seconds, Timekeepers insert a leap second so civil time does not wander away from the sky.
Since leap seconds were introduced in 1972, they have always been added, not removed, because the long term trend has been a gradual slowdown. Those daily fractions of delay accumulate, so international authorities decided to patch the gap by occasionally inserting an extra second into Coordinated Universal Time, or UTC. In fact, twenty-seven times we have actually added a leap second to make up for the slower spin, as popular explainers on social video remind viewers. The system was built on the assumption that Earth’s rotation would keep stretching out, not snap back the other way.
Why Earth is suddenly racing ahead of our clocks
Over the past few years, that assumption has been shaken. Measurements of Earth’s rotation show that since 2020, the planet has been spinning a little faster, enough that some days are among the shortest ever recorded. Scientists tracking these changes report that Earth has been moving faster than it has in roughly half a century, with some years predicted to have average days slightly under 86,400 seconds, a pattern highlighted in widely shared measurement posts. That acceleration is what now threatens to push atomic time ahead of astronomical time in the opposite direction.
Researchers agree that there is no single culprit. For billions of years, the moon has been gently tugging at Earth and slowing its rotation, so days 200 million years from now will be longer still, as explained in detailed breakdowns of Earth and the moon. Yet on shorter timescales, internal and surface processes can override that slow brake. Analyses of the difference between atomic time and Earth’s spin show that, Finally, over the past 30 years, the rotation of the crust has accelerated, likely linked to flows in the outer core of the earth. There are a number of reasons the planet may be changing its speed this way, but it is not yet clear just which explanation is responsible this time, as recent There reporting notes.
Climate change, melting ice and the tug-of-war on planetary spin
On top of deep interior dynamics, climate change is now part of the story of time itself. As Greenland’s ice sheet near Baffin Bay and Antarctica lose mass, water flows into the oceans and redistributes weight around the globe, subtly altering the planet’s spin, which affects global timekeeping, a link that recent coverage of Greenland and Earth makes explicit. One detailed study found that, On the other hand, apart from tidal friction, another factor slowing the spin was the increased melt from the Greenland ice cap but also from Antarctica, according to geophysicist Duncan Agnew’s analysis of On the Earth. That means global warming is not simply speeding the planet up or slowing it down; it is reshaping how mass is arranged, with competing effects on rotation.
Other work has gone further, arguing that Negative Leap Seconds, Global Warming, Slowing Down Time and related changes in the Earth system are now intertwined, as one recent analysis of Global warming and Earth’s spin puts it. Climate change, again and seemingly always, may play a role too, with NASA funded studies finding that since 2000, melting glaciers and shifting water have measurably altered rotation, a point underscored in Climate reporting. I read this as a planetary tug-of-war: the moon is lengthening our days over geological time, while human driven warming is, in the near term, both slowing and speeding different aspects of the spin by moving ice and water around.
The first negative leap second and the risk to digital infrastructure
If current trends hold, world timekeepers may soon have to consider subtracting a second from our clocks because the planet is rotating too quickly. Scientists tracking Earth’s spin say that by around 2029, the gap between atomic time and solar time could grow large enough that Clocks might have to skip a second to keep up, a possibility flagged in recent Clocks coverage. For the first time in history, world timekeepers are openly discussing reducing one second from Coordinated Universal Time as Earth’s rotation speeds up, a rare and unprecedented move highlighted in a widely shared Why explainer that likens the planet to a skater pulling in their arms to spin faster.
Technically, deleting a second is harder than adding one. When Should the Earth’s rotation get out of sync with atomic clocks, a leap second can be added or subtracted to ensure they remain aligned, and since 1972 only 27 leap seconds have been added to the year, as one detailed Should the Earth summary notes. Software and hardware are built around that pattern. If there is a change in the usual direction, not all devices can sync, as one analysis of Negative Leap Seconds warns. Past positive leap seconds have already caused glitches for services like Cloudflare and Reddit; a negative leap second would force engineers to rewrite assumptions baked into everything from stock exchange servers to GPS receivers.
From satellites to smartphones, who feels the missing second?
The stakes are not abstract. Atomic clocks underpin the timing signals that let GPS satellites tell your phone where it is, and even tiny mismatches can translate into meters of error on the ground. Earthquakes, atmospheric winds, ocean currents, and remarkably even human activity itself, all act often and unpredictably to redistribute the planet’s mass and the orientation of its spin axis, complicating the job of keeping track of spacecraft as Earth’s water alters its spin, as mission controllers at the European Space Agency explain in their Earthquakes briefings. When the underlying rotation itself shifts, all of those corrections have to be updated, or satellites risk drifting off their predicted paths.
At the same time, climate change is altering how scientists measure time. Shifts in the distribution of mass at the surface can affect Earth’s spin, which in turn forces timekeepers to rethink how often they adjust UTC, a link explored in depth in coverage of how Shifts in Earth are changing timekeeping. I see a parallel with how inflation quietly erodes the value of money: most people will never notice a millisecond shaved off a day, but the infrastructure that prices trades, routes data packets, and steers aircraft depends on those tiny units staying consistent. If time starts to “inflate” or “deflate” relative to Earth’s spin, the hidden plumbing of the digital world has to be rebuilt.
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*This article was researched with the help of AI, with human editors creating the final content.
