“Time is what we want most, but use worst,” William Penn once wrote. But for the first time in the annals of modern times, the very fabric of timekeeping is being threatened by no human weakness, but by the planet itself. The Earth will complete its rotation 1.34 milliseconds shy of the normal 24 hours on July 22, 2025, one of the shortest days since atomic clocks were developed. Though subtle to human perception, these tiny milliseconds are raising eyebrows and alarm among scientists and engineers tasked with synchronizing the clocks of the world.
It is not an isolated occurrence. The Earth has repeatedly broken its own speed records over the past half-decade, and its shortest day ever recorded was on July 5, 2024, when the planet spun 1.66 milliseconds faster than usual. This acceleration trend is unexplainable, especially since Earth’s long-term motion has been towards deceleration, primarily due to tidal friction generated by the moon. “The cause of this acceleration isn’t balanced,” Leonid Zotov, leading Earth rotation specialist at Moscow State University, told Timeanddate.com. “Most scientists believe it is something inside the Earth. Ocean and atmospheric models don’t explain this huge acceleration.” Ocean and atmospheric models do not factor in this colossal acceleration.
Historically, the moon’s gravitational force has acted as a brake, slowing Earth’s days by roughly 2.3 milliseconds per century. This tidal friction, supplied mostly by ocean bulges, has been the dominant force behind Earth’s rotational evolution for more than billions of years. Other forces are now said to be acting by recent studies. Below the surface, the physics of the liquid outer core are charged with redistributing angular momentum, fine-tuning the speed at which the mantle and crust rotate. Earth’s speeding up because its hot liquid core a large ball of molten fluid acts in unpredictable ways, with eddies and flows that vary, explained Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego NASA-funded study details how climate is leading Earth’s rotation to slow down. But it does not stop there with the core.
The redistribution of mass through the melting of polar ice and rising sea levels, driven by worldwide climatic change, is also a dominant force. Water migrates from poles to equator as ice sheets and glaciers melt, contributing to the equatorial bulge of the Earth and its moment of inertia. This is akin to an ice skater outstretching their arms to slow down their spin rate, which acts contrary to the recent spin rate of acceleration, but not enough to fully cancel it out. As indicated in a recent study, “climate change could become the new dominant factor,” to overtake the moon’s role of adding to the length of the day if emissions are not restricted climate change could become the new dominant factor. The effects of these small changes extend far beyond cognitive interest.
Modern society relies on atomic clocks equipment so precise they would gain neither nor lose one second over millions of years to determine Coordinated Universal Time. 27 leap seconds have been added to UTC since 1972 to accommodate the difference between atomic time and slightly less uniform astronomical time controlled by Earth’s rotation. “We invited the director to come and witness the death of the astronomical second and the birth of atomic time,” recalled Louis Essen, who constructed the first stable enough atomic clock to be used as a time standard the advent of atomic time. Now, for the first time, timekeepers are discussing a negative leap second removing a second from UTC, rather than adding one.
This is an extraordinary circumstance and a large deal, stated Duncan Agnew. “It’s not a large Earth rotation change that’s going to cause some catastrophe or anything, but it is something significant. It’s yet another indication that we’re in a very unusual time.” If current trends persist, this adjustment may be required as soon as 2029, a move never before attempted in the history of precision timekeeping. The problem is not purely technical.
Numerous computer systems, including those supporting global navigation satellites and financial networks, are designed to accommodate the addition of leap seconds, yet not their removal. “It’s likely to be tougher to skip a second because software programs are designed to add, not subtract time,” former U.S. Naval Observatory director of time Dennis McCarthy explained. Google and Amazon have already constructed their own hacks, “smearing” leap seconds taken over hours to avoid disruptions tech firms using leap-second workarounds. As researchers continue to track Earth’s spin with atomic clocks and satellite observations, the dynamic interaction between celestial mechanics, geophysics, and human-forced change is becoming increasingly evident.
“In barely 100 years, human beings have altered the climate system to such a degree that we’re seeing the impact on the very way the planet spins,” said Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory. The world’s clocks, once a concern of the celestial determinate, now intimately rely upon the dynamic balance between Earth’s oceans, atmosphere, and molten core.
