Every few years since 1972, computers, financial institutions, communications networks and other systems have had to reset their clocks by a single second.
On Thursday, the international authority on timekeeping voted to abolish this contentious quirk of global synchronization. The change will take effect in 2027.
In a meeting of officials from the International Bureau of Weights and Measures, or B.I.P.M., in France, the group axed the leap second, which had been established to align our clocks with Earth’s rotation.
The practice was similar to that of the leap year, when an extra day is added to our calendars to keep us in sync with Earth’s orbit around the sun. Leap years are predictable, happening every four years. But leap seconds account for seemingly random irregularities in how fast Earth rotates, which has been causing trouble for timekeepers, including the looming possibility of needing a “negative leap second” because Earth’s spin is getting faster.
“This decision has become particularly timely because recent changes in the Earth’s rotation have raised the possibility of a first-ever negative leap second, which has never been experienced and that could create additional risks for critical infrastructures,” Noël Dimarcq, president of the B.I.P.M.’s committee for time and frequency, said in a statement.
In lieu of the added second, humankind will wait until an entire hour is needed to bring us back into synchronization with the planet’s spin. This new practice is “just easier” to keep track of, said Patrizia Tavella, director of the bureau’s time department.
It’s unclear when this extra hour may be needed, but it might not happen for centuries.
“It’s really just taking the problem well, well into the future,” said Andrew Novick, an electrical engineer and manager of the time and frequency division at the National Institute of Standards and Technology, or NIST, in Maryland. “It could be thousands of years before this leap hour happens.”
For much of history, “the Earth was the best available clock,” Duncan Agnew, a geophysicist at Scripps Institution of Oceanography in California, explained during the meeting. Keeping time mostly involved ways of dividing how long it takes Earth to complete one rotation on its axis: Earth rotates at about 1,000 miles per hour, on average, so a day equals 86,400 seconds.
By the mid-20th century, humans needed more timekeeping precision to support the advent of computers, international communications systems and space exploration. Enter atomic clocks, which measure time by tracking the tiny changes atoms undergo at regular intervals. These ultraprecise clocks ushered in a global way of keeping time: Coordinated Universal Time, or U.T.C., which was adopted in 1963.
“You only know how good your clock is when you get a better clock,” Mr. Novick said.
Sometimes gravitational interactions with other celestial bodies or the movements of air, water and rock on our dynamic planet can affect its rotation speed. Once clocks became more precise, these subtle shifts in Earth’s rotation became more noticeable, which is why we adopted the leap second. In recent decades, though, the added second posed two main problems for our digital systems.
For starters, it can be a hassle to change the time by a single second and could even cause time-linked systems to temporarily shut down. Some tech giants such as Google adopted a “leap smear” instead, temporarily slowing down their clocks rather than adding the leap.
What’s more, Earth’s rotation isn’t always lagging behind. The same forces that slow it down can also work in reverse, meaning that one day we might have a negative leap second. And while most of the digital systems that rely on U.T.C. are prepared to jump ahead, far fewer have the code built in to handle a move backward in time.
Since the 2000s, Earth’s rotation has been speeding up, driven by a combination of changes to Earth’s core and surface phenomena like the melting polar ice caps. This makes the need for a negative leap second more likely, and, in part, drove timekeepers to their decision to end the leap second.
The vote was not unanimous. Delegates from Russia, whose satellites often have a built-in system for handling leap seconds, voted against the measure, and those from Belarus abstained.
The differential between our clocks and Earth’s rotation is currently hovering around zero, meaning the next leap second could have easily been a negative one, Mr. Novick said. Still, Mr. Novick and other scientists said they will miss this little idiosyncrasy in the way we keep time.
“It’s the one time you could almost assure that NIST would be on the news,” Mr. Novick said, “because it gets people thinking about the Earth’s rotation, and about time.”
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