It doesn’t happen often that the Earth puts timekeepers in their place, but on August 5, 2025, the world is set to do just that rotating so fast that the day will be 1.34 milliseconds shorter than a regular 24 hours. Most folks won’t even register the momentary loss, but this tiny change is a life-and-death moment for scientists and engineers who rely on the unyielding accuracy of atomic clocks and the global timekeeping system.
The tale starts deep within our very foundation, in the agitated core of the world. Geophysicists have mapped the movement of the inner core of Earth for decades, a solid ball of metal surrounded by a turbulent ocean of liquid iron. Latest seismic studies have yielded unequivocal proof that the inner core started slowing down about 2008, traveling slower than Earth’s surface. This slowing down is not merely of interest to seismologists it has a direct, though minute, impact on the length of our days. According to Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography, “The core is what changes how fast the Earth rotates on periods of 10 years to hundreds of years. The core has been slowing down for the last 50 years, and as a result, the Earth has been speeding up.”
But the core is not all there is to it. The atmosphere also has a role, seasonal in nature. During Northern Hemisphere summers, atmospheric winds and jet streams change, redistributing angular momentum between air and planet in subtle ways. The moon’s gravity introduces another complication: when the moon is in its most extreme positions away from the equator, its drag on Earth’s rotation is less, and the planet can spin a fraction faster. These factors combine to produce seasonal fluctuations in day length, often peaking in July and August as seen in recent years.
Such changes are imperceptible to daily life, but for the global timekeeping community, they are anything but trivial. The world’s clocks are set not by the sun or the stars, but by theultra-precise oscillations of atoms a revolution that began in the 1950s with the advent of atomic clocks. These devices, now numbering around 450 in the global ensemble, define the Coordinated Universal Time (UTC) standard. In the words of David Gozzard, an experimental physicist at the University of Western Australia,“If the clocks are thrown off even a tiny amount, it could also throw off computers, servers, GPS signals, and other networks that rely on accurate times.”
To make astronomical time and atomic time coincide, the International Earth Rotation and Reference Systems Service (IERS) adds a “leap second” to UTC from time to time. Since 1972, 27 leap seconds have been added, all positive, to account for the slowdown of the Earth. With the recent tendency toward shorter days, however, the possibility of a “negative leap second” taking one away from UTC has arisen. “There’s never been a negative leap second,” states Judah Levine, a physicist at the National Institute of Standards and Technology. “But the probability of having one between now and 2035 is about 40%.” This prospect has been controversial for engineers, as the historical shift could be disorienting to key systems, evoking the Y2K fears and challenging the robustness of the world’s networks.
In the midst of these challenges, the technology of timekeeping itself is in the middle of a revolution. Next-generation optical atomic clocks, including those built at JILA, are now 100 times more accurate and stable than the best cesium fountain clocks. These clocks probe the vibrations of atoms like strontium and ytterbium using visible light and subdivide the second into increasingly finer increments. “This clock is so precise that it can detect tiny effects predicted by theories such as general relativity, even at the microscopic scale,”physicist Jun Ye says. The new clocks are so sensitive that it takes less than a millimeter to raise or lower them to detect the warping of time by gravity a tribute to the brilliance of their design as well as to the fineness of the universe they time with record-breaking accuracy.
And even as technology improves, the relationship between Earth’s geophysical processes and human timekeeping is still a delicate dance. Melting ice caps, changing winds, and the moon’s constant pull all combine to keep the day’s length in a state of flux. It’s an extremely difficult problem, to untangle the different contributing factors, comments Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory. The outcome is a world in which millisecond precision matters and in which the search for flawless time is, appropriately enough, never completed.
