The world’s timekeepers will meet Thursday near Paris to decide whether to eliminate the “leap second,” which could disconnect the clocks we use from the Earth’s rotation and realign time scales. The world’s clocks are designed on the idea that the Earth completes one rotation on its axis in exactly 24 hours. But it actually wobbles, making the time it takes the Earth to spin less accurate. And over time, the Earth’s rotation is gradually slowing down. To compensate for that, since 1972, timekeepers have added a leap second every few years (the last one was added in 2016) to keep clocks and the Earth’s rotation in sync. But pressure has mounted to eliminate the leap second, as it complicates matters for financial markets and data centers, which rely on millisecond-precise timing. second? Could it be discarded? And how might it change our understanding of time? Here’s what we know: What is a leap second? The clocks operate according to Coordinated Universal Time (UTC), the global standard that has been based on high-precision atomic clocks for more than seven decades. Atomic clocks keep time by observing the vibration of atoms. But the Earth’s rotation works independently of atomic clocks. A leap second is a second that is periodically added to UTC to synchronize it with the Earth’s rotation. The first leap second was added to world clocks in 1972. 27 leap seconds have been added so far and the previous leap second was added on December 31, 2016. No more leap seconds have been added since then. According to the National Institute of Standards and Technology (NIST), a non-regulatory federal agency of the United States Department of Commerce, leap seconds occur in late December or late June. NIST also cites differences between the precision of the atomic second and the Earth’s rotation to explain the need for a leap second. “The Earth’s rotation speed is not constant… This gradual decrease in the rotation rate is causing the duration of the mean solar second to gradually increase relative to the atomic second,” NIST added. companies, financial markets and other businesses that depend on precise times. In 2012, LinkedIn and Reddit crashed after adding a leap second. The same year, ticket bookings for Qantas, the Australian airline, collapsed after the leap second was added. Three years later, the Australian Securities and Investments Commission also issued a warning against it. And in July 2022, Meta engineers said: “Every leap second is a major source of pain for people managing hardware infrastructures.” Faced with mounting pressure, the International Bureau of Weights and Measures, which has the French acronym BIPM, voted in 2022 to eliminate the leap second by 2035. But countries like Russia have wanted to delay the implementation date until 2040. Russia incorporates leap seconds directly into its GLONASS satellite navigation system, so delaying the elimination of leap seconds would give Russia time to update its satellite systems. Despite Moscow’s concerns, the BIPM’s 69 member states will meet on Thursday in Versailles, outside Paris, to debate and vote on a resolution to eliminate the leap second by 2027. What will be debated on Thursday? According to the BIPM, timing authorities from countries around the world will discuss three proposals at Versailles. First, they will consider whether atomic clocks are still the most accurate device when it comes to measuring what constitutes a second. In fact, optical frequencies are now more precise than atomic clocks. If one proposal to change the way time is measured is accepted, the second could have a new definition, characterized by optical frequencies rather than vibrations of atoms. The second proposal, the most significant, concerns getting rid of the leap second. And finally, the third proposal on the table is a pitch to synchronize time measurements on Earth with a new time standard that NASA is developing for the Moon. The accepted proposals are expected to come into force on May 20, 2027, World Metrology Day. What happens if the leap second is discarded? If the leap second is discarded, then it means that our clocks will no longer be linked to the Earth’s rotation. According to NIST, if this happens, “then adjustments would have to be made to timestamps and legacy equipment and software that synchronize with UTC for astronomical observations.” Server outages and network failures in financial markets will be eliminated with the abandonment of leap seconds. But over a period of centuries, the gap between Earth’s cycle and our clocks will become more apparent: by 2100, clocks will be 54 seconds ahead of Earth. In another two millennia, that gap would have widened to several hours. The result? Sunrise and sunset times that would be out of sync with the natural rhythms of people today.
