Scientists have just voted to abolish leap seconds

Leap seconds, previously used to keep the world’s clocks in sync with the Earth’s rotation, are to be abolished. Instead, universal time will be allowed to drift up to an hour from astronomical time, before it is corrected. Representatives from 69 countries voted for the change on 15 October at a meeting of the International Bureau of Weights and Measures (BIPM) in Versailles, France.

As the gap between clock time and astronomical time slowly grows, the effect will eventually be “like living on daylight savings time”, but permanently, says Emily Akkermans, curator of time at the Royal Observatory in Greenwich, London.

For most of history, our clocks have been set by the apparent position of the sun in the sky. But in 1967, after the development of super-accurate atomic clocks, physicists defined an international time standard called coordinated universal time (UTC), based instead on the speed of a particular vibration of the caesium-133 atom.

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Natural fluctuations in the Earth’s rate of rotation meant that periodic corrections were needed to keep universal time in sync with the sky. As the rotation slowed, physicists added extra leap seconds. Twenty-seven have been added to UTC since they were introduced in 1972.

Now that strategy is to be abandoned. This is partly because technological systems, from stock markets to telecommunications to GPS, have become much more reliant on precise timings. That means introducing leap seconds is more complex, potentially risking failures of mobile phone networks or electricity grids. Previous leap seconds have caused websites to crash and disrupted airline booking systems, says Setnam Shemar at the National Physical Laboratory (NPL) in Teddington, UK.

“Time is now so much more important and integrated into these systems,” adds Akkermans. “What once worked has become more problematic.”

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What’s more, the Earth’s rotation has recently begun to accelerate slightly, raising the daunting possibility of needing “a first-ever negative leap second”, said Noël Dimarcq, who heads the BIPM’s committee on time and frequency, in a statement. The practical consequences of such a jump backwards in time are hard to predict.

Physicists have been debating what to replace the leap second with for the past 30 years. At the BIPM meeting, they finally voted for a “leap hour”, which means that UTC will be allowed to diverge from astronomical time for up to an hour before being corrected. That should ensure an uninterrupted flow of time for “several centuries”, said Dimarcq. The change will take effect from 20 May 2027.

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Shemar emphasises that the effects involved are very small. “It’ll be many hundreds or even thousands of years before a leap hour is needed.” He adds that physicists will still be accurately tracking the Earth’s rotation, to provide information for astronomy and celestial navigation.

The leap hour isn’t the only temporal innovation being planned, though. The BIPM meeting also asked for proposals on a new definition of the second – needed because optical clocks are now much more accurate than the caesium clock that time is currently based on – and called on international agencies to start considering how to define a universal time standard for the moon. Overall, the changes “will have a profound impact on future timekeeping both on the earth and other planets,” said Dimarcq.