Leap seconds, previously used to keep the worldās clocks in sync with Earthās 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 in Versailles, France.
As the gap between clock time and astronomical time slowly grows, the effect will eventually be ālike living on daylight saving timeā, but permanently, says , 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 Earthās rate of rotation meant 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 timekeeping. That means introducing leap seconds is more complex, potentially risking the failure of mobile phone networks or electricity grids. Previous leap seconds have caused websites to crash and disrupted airlinesā booking systems, says at the National Physical Laboratory (NPL) in Teddington, UK.
āTime is now so much more important and integrated into these systems,ā says Akkermans. āWhat once worked has become more problematic.ā
Whatās more, Earthās rotation has recently begun to accelerate slightly, raising the daunting possibility of needing āa first-ever negative leap secondā, said , who heads the BIPMās 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 āleap hourā, 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 āseveral centuriesā, said Dimarcq. The change will take effect from 20 May 2027.
Shemar emphasises that the effects involved are very small: āItāll be many hundreds or even thousands of years before a leap hour is needed.ā He adds that physicists will still be accurately tracking Earthās rotation to provide information for astronomy and celestial navigation.
The leap hour isnāt the only temporal innovation being planned, though. The BIPM meeting also asked for proposals on a new definition of the second ā needed because optical clocks are now much more accurate than the caesium clock that time is currently based on ā and called on international agencies to start considering how to define a universal time standard for the moon. Overall, the changes āwill have a profound impact on future timekeeping both on Earth and other planetsā, said Dimarcq.
