By Katie Hunt, CNN
(London) — When the sun hits its highest point over the meridian line that travels north and south from the 351-year-old Royal Observatory Greenwich in London, it’s noon Greenwich Mean Time, a tangible and physical reference point that helped establish the world’s time zone system.
Timekeeping has long relied on Earth’s spin and location of the sun or moon in the sky. Before observatories such as Greenwich, humans used sundials — essentially a stick in the ground that casts shadows as the sun moves overhead. But this ancient link between Mother Earth and today’s clocks looks set to be severed.
International timekeepers are on track to vote Thursday on a proposal to eliminate what’s known as a leap second — a tiny, but many say important, link between Earth’s rotation and the world’s time standard used in critical infrastructure such as telecommunications, power grids and computers.
Inserted into global clocks 27 times since 1972, the leap second essentially allows two time systems to run in sync: astronomical time, long anchored by Earth’s rotation and measured by observatories, and Coordinated Universal Time, or UTC, the global standard that’s set by ultraprecise atomic clocks first developed in the 1950s. These cutting-edge, lab-based clocks rely on tracking the natural vibrations — or ticks — of atoms.
The two time-telling methods differ because Earth does not rotate at a steady pace but wobbles, essentially making it a “lousy” clock, said Jeff Sherman, a group leader of the National Institute of Standards and Technology’s Time and Frequency Division in Boulder, Colorado.
This wobbling stems from various factors, such as the moon’s gravitational pull, shifts deep in the planetary core, and the movement of water and ice on Earth’s surface — an effect that’s exacerbated by the melting of the polar ice caps due to the climate crisis.
“We have to make a choice on whether or not the most important thing is a sense of time given by objects in the sky directly above, say, the Greenwich longitude,” he said referring to the Greenwich meridian that marks 0 degrees longitude. “Or you decide to follow what you believe are more stable, laboratory, man-made clocks,” Sherman added.
Leap seconds have served as the compromise between the two time-telling systems. Most people never notice them — unlike a leap year, which adds an extra day to February every four years to account for the discrepancy between the Gregorian calendar used in most parts of the world and the time it takes Earth to complete one orbit around the sun.
But the addition of leap seconds, which happens at irregular intervals and is announced with only six months’ notice, creates problems for operators of data servers, high-frequency financial trading systems and other infrastructure that rely on precise timekeeping to work properly.
The extra second crashed Reddit and LinkedIn and knocked out Qantas’ booking system in 2012. And when the most recent leap second was added at the end of 2016, customers of cloud computing provider Cloudflare briefly experienced outages or delays after its servers failed to handle the introduction of a leap second properly. Big tech players such as Meta have been calling for its demise, describing the leap second as “a major source of pain.”
Why lose the leap second?
The 69 member states of the International Bureau of Weights and Measures, an intergovernmental organization that determines measurement standards, will meet this week in Versailles near Paris. The group, known by its French acronym BIPM, will vote on a resolution to ditch the leap second, a move that would take effect in May 2027. A majority needs to support the proposal, said Patrizia Tavella, director of the time department at BIPM.
When atomic time was first introduced, Tavella said, it was important to keep it tethered to astronomical time because navigational tools still relied on traditional celestial methods and timekeeping. With the advent of GPS, however, that was no longer the case. “This correction in time has become a serious issue for modern applications,” she added.
Members of BIPM have discussed the switch to what Tavella called Continuous Coordinated Universal Time for more than 20 years, she noted. At the group’s last meeting in 2022, member countries voted in favor of a resolution that called for a plan to phase out the leap second on or before 2035.
In the past, some countries, such as Russia and the United Kingdom, have opposed ditching the leap second, she noted. Russia’s 2022 opposition stemmed from the fact that its Global Navigation Satellite System, or GLONASS, incorporates leap seconds, unlike other satellite navigation systems, such as the United States’ GPS and the European Union’s Galileo, that do not. The United Kingdom was once opposed because of its cultural attachment to the foundational role that Greenwich played in the history of time, but its stance has since changed, she added.
Urgent timekeeping debate
Since the world’s timekeepers last met in 2022, the debate over whether to abolish the leap second has taken on an urgency because of the prospect of something unprecedented: It will soon be necessary to subtract rather than add one second, known as a negative leap second, according to experts.
The long-term trend of Earth’s rotation is slowing, but in the near term it appears to be speeding up, potentially requiring a subtraction of a second from Coordinated Universal Time to keep astronomical and atomic time matching.
A 2025 report by the International Earth Rotation and Reference Systems Service, a consortium headquartered in Frankfurt, Germany, tasked with tracking Earth’s rotation, estimated that the likelihood of a negative leap second would rise sharply in the coming years — hitting a probability of 30% by 2035, according to BIPM.
Deleting a second has never previously been attempted and could create havoc in today’s digitally time-stamped world. BIPM said the operators of critical infrastructure are “largely unprepared” for such a move. “The preparation would create, for the industries that rely on time synchronization, a need for financial investment, whose amount is estimated to be similar to their preparations for the millennium bug,” the organization said, referring to Y2K, a computer flaw that programmers thought might cause severe systemic problems after December 31, 1999.
However, not everyone agrees that it’s necessary to ditch the leap second.
The technical challenges of incorporating leap seconds, whether negative or positive, into digital systems were overstated, according to Marek Ziebart, a professor of space geodesy at University College London who said there was value in keeping the world’s fundamental timescale aligned with Earth’s rotation.
“We have a system in place that uses leap seconds. It works. It maintains the timescale that all of us use on a day-to-day basis,” Ziebart said. “If you abolish the leap second, a lot of things will have to change. People will have to check their software, redesign it, fix it.”
He added that it was also “intellectually comforting” to maintain a timescale tied to Earth’s rotation. “There’s an intrinsic relationship for human beings between rotation and time,” he said. “It relates to us as animals. We’re driven by these periodicities of the sun rising, the sun setting.”
Enter the ‘leap hour’
Instead of juggling positive and negative leap seconds, the BIPM resolution essentially proposes that atomic time would be allowed to drift gradually away from astronomical time. Doing so means official timekeeping would become independent of Earth’s rotation, and another leap second would never be added to clocks.
However, the proposal does recommend that the maximum difference between atomic and astronomical time should not exceed 3,600 seconds or one hour. It would take more than 3,000 years for such a difference to occur, BIPM’s Tavella said.
The move essentially punts the problem to future generations, and they would have to decide whether or not to take a “leap hour,” said Louise Devoy, senior curator of the Royal Observatory Greenwich. Timekeeping could look dramatically different then. Perhaps today’s atomic clocks, tuned to the steady vibrations of cesium atoms, could one day seem as quaint as sundials.
“I can’t imagine us changing it so drastically that it then becomes noticeable in our daily lives. We still have our daily routines, our natural circadian rhythm, our need to be in daylight,” Devoy said.
For most, herself included, ditching the leap second would be a behind-the-scenes shift that would make little material difference. “It’s just the way that our lives have changed,” she said. “We’re so dependent on technology now.”
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