We may be about to call time on the “leap second” which, for decades, has kept Coordinated Universal Time – the global standard that regulates clocks and time zones – in sync with Earth’s daily rotation.
The world’s timekeeping body already had a plan to phase out the leap second by 2035 “but now there is a spanner – or spinner – in the works,” said the Financial Times. The Earth has been rotating unusually rapidly for the past decade, meaning there’s a chance that, before 2035, we’d actually need a negative leap second – deleting a second, rather than adding one. This has never been attempted before and could “create mayhem in the digital time-stamped continuum”. So, the governing body of the International Bureau of Weights and Measures is this week urgently considering condemning the leap second “to an earlier grave”.
How do we measure time?
“For millennia, a day has been defined as one full rotation of the Earth,” said physics and astronomy professor Vahe Peroomian on The Conversation. Ancient Egyptians were the first known civilisation to divide day and night into twelve parts each, establishing the 24-hour day.
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In the second century BC, the astronomer Ptolemy laid the foundations of a system that divided an hour into 60 minutes, and a minute into 60 seconds. But it took many centuries before devices, such as 17th-century pendulum clocks and 20th-century quartz watches, “became accurate enough” to measure seconds, said BBC Future.
What’s the problem with astronomical timekeeping?
The speed of the Earth’s rotation can fluctuate, so defining a second as a fraction of a solar day is imprecise. In 1967, the General Conference on Weights and Measures adopted a new definition of a second based on the cesium-133 atom, which oscillates at “a remarkably steady rate”, said Peroomian. One second was defined as 9,192,631,770 oscillations. Atomic clocks use cesium-133 to give us Coordinated Universal Time (UTC): the super-precise global standard used by telecoms, computer networks, international businesses and power grids.
So what are leap seconds?
Atomic clocks run at a fixed rate but – apart from right now – Earth’s rotation has been gradually slowing, primarily due to the Moon’s gravitational pull. To keep UTC in sync with Earth’s rotations, timekeepers invented “the leap second” in 1972. This “interloper” allows the two systems of time – astronomical and UTC – to “run in parallel”, said the Financial Times. Once the lag reaches a certain point, a leap second is inserted to “yank them back in alignment”. At irregular intervals, 27 leap seconds have been added to global clocks since 1972 – the last in 2016.
Why are they being phased out?
Leap seconds have become “increasingly difficult to deal with in the computer age”, said ABC News. They can disrupt computer operating systems, which assume a day is always 86,400 seconds long, resulting in glitches and outages. But the prospect of the first negative leap second poses an even greater risk. “Many computer programmes assume all leap seconds are positive”, Setnam Shemar, the UK’s principal timekeeping scientist, told the FT. Removing a second could “result in a major technology failure in telecommunications or electricity grid networks or satellite navigation systems”.
If they do ditch the leap second, the world’s timekeepers will vote on introducing a “leap hour” instead. This could take “a millennium or more” to accumulate and, by then, life may be so different, there may be no need to adjust the lag at all. All this change “would be a practical win for tech overlords, digital gatekeepers and official clock-watchers” but it could be “something of a cultural loss for our stargazing species, detaching human timekeeping from its ancient, astronomical roots”.
