The leap second has been cancelled. It’s about time

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 synchronisation. The change will take effect in 2027.

In a meeting of officials from the International Bureau of Weights and Measures, or BIPM, 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,” Noel Dimarcq, president of the BIPM’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 synchronisation 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 1000 mp/h (1600 km/h) 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 ultra-precise clocks ushered in a global way of keeping time: Coordinated Universal Time, or UTC, which was adopted in 1963.

“You only know how good your clock is when you get a better clock,” 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.