It’s Official: The Leap Second Will Be Retired (a Decade from Now)

The time has come — or will come, in 2035 — to abandon the leap second.

So voted the member states of the international treaty governing science and measurement standards, at a meeting in Versailles, France, on Friday. The near-unanimous vote on what was known as Resolution D was met with relief and jubilation from the world’s metrologists, some of whom have been pressing for a solution to the leap second problem for decades.

“Unbelievable,” Patrizia Tavella, director of the time department of the International Bureau of Weights and Measures, known as B.I.P.M. from its French name and based outside Paris, wrote in a WhatsApp message shortly after the vote. “More than 20 years of discussion and now a great agreement.” She added that she was “moved to tears.”

The United States was a firm supporter of the resolution. “It feels like a historic day,” said Elizabeth Donley, chief of the time and frequency division of the National Institute of Standards and Technology, or NIST, in Boulder, Colo. “And I wish I was there. There’s probably a lot of celebrating being done in style.”

The leap second has caused trouble since its inception 50 years ago. It was devised as a way to align the international atomic time scale, in use since 1967 and derived from the vibration of cesium atoms, with the slightly slower time that Earth keeps as it rotates. In effect, whenever atomic time is one second ahead, it stops for a second to allow Earth to catch up. Ten leap seconds were inserted into the atomic time scale when the fudge was unveiled in 1972. Twenty-seven more have been added since.

Those extra seconds were tricky to insert in 1972; today, the technical issues are gnarly. For one, it’s hard to predict exactly when the next leap second will be needed, so computing networks cannot prepare for orderly, regular insertions. Different networks have developed their own, uncoordinated methods of incorporating the extra second.

Moreover, modern global computing systems have become more tightly intertwined and more reliant on hyper-precise timing, sometimes to the billionth of a second. Adding the extra second heightens the risk that those systems, which are responsible for telecommunication networks, energy transmission, financial transactions and other vital enterprises, will crash or fail to synchronize.

As a result, unofficial time systems have slowly begun to displace the world’s official international time, Coordinated Universal Time, or U.T.C. Eliminating the leap second is seen as a way of preserving adherence to U.T.C. by making it a continuous time scale rather than one that is episodically interrupted.

“The most important issue is the preservation of the concept that time is an international quantity,” said Judah Levine, a physicist at NIST. He called the Versailles decision “an incredible step forward.”

Russia voted against the resolution; Belarus abstained. Russia has long sought to delay abandonment of the leap second because its GLONASS global navigational satellite system incorporates the extra seconds, unlike other systems such as GPS, which is operated by the United States. With Russia’s concerns in mind, the leap second is not scheduled to be dropped until 2035, although it could happen sooner.

Resolution D calls for U.T.C. to go uninterrupted by leap seconds from 2035 until at least 2135 and for metrologists to eventually figure out how to reconcile the atomic and astronomical time scales with fewer headaches. The international time standard would be severed from time as told by the heavens for generations to come.

But rejoining those two time scales was imperative, said Rev. Pavel Gabor, an astrophysicist and the vice director of the Vatican Observatory Research Group in Tucson, Ariz. He said that atomic timekeeping was just one example of how the world was becoming incomprehensible to the average person, and that scientists had a responsibility to help people feel in control of their lives.

“I think sensitivity to this mistrust of elites, mistrust of experts, mistrust of science and institutions, that’s something that’s a very real problem in today’s world,” he said. “And let’s not contribute to it.”

Steps remain in the elimination of the leap second. Although B.I.P.M. is responsible for universal time, the International Telecommunication Union, or I.T.U., is responsible for transmitting it. The I.T.U.’s World Radiocommunication Conference in Dubai, United Arab Emirates, will also vote on the issue next year. Felicitas Arias, the former director of the time department at B.I.P.M. and now a visiting astronomer at the Paris Observatory, said that negotiations between the two organizations convinced her that the I.T.U. will support the Versailles vote.

“Now we see really closer the moment to have continuous time,” she said, applauding the vote on Friday. “And this is something we have been dreaming about for a long, long time.”

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