After decades of discussion, quibbling and worrying about clocks in computer systems, the world’s official timekeepers have voted to replace what many viewed as a tiresome correction: the leap second. It’s time, they say, for the leap hour. Delegates from 69 countries cast their votes on Thursday at a conference in France of the International Bureau of Weights and Measures, which ensures the world sticks to the same standards of time, weight, distance and more.
The decision to adopt the leap hour from next year aims to reduce the risk of computers and other technical systems tripping over as their software grapples with minutes lasting 61 seconds – and, worse, 59 seconds – instead of a round 60. The more familiar leap year was introduced more than 2,000 years ago to ensure that the calendar year did not fall out of step with the time that the Earth takes to circle the sun. Adding a day to the end of February every four or so years does the job, because Earth orbits the sun every 365 and a quarter days.
Leap seconds are a much more modern invention. In 1955, Louis Essen and Jack Parry at the NPL built the world’s first practical, high-precision caesium atomic clock. The device revolutionised timekeeping, and in 1967 the astronomical definition of a second, based on the rotation of the Earth, was abandoned.
Ever since, the formal definition of one second has been 9,192,631,770 vibrations of the caesium atom. Atomic clocks are exceptionally stable and precise. Their timekeeping has become critical for modern computer systems, communications, satellites, trading systems and electricity grids.
But the Earth’s rotation is far less reliable. Its spin is affected by the planet’s molten core, the shift of the oceans, global jet streams and other natural forces. Broadly, Earth’s rotation is slowing down: when an asteroid doomed the dinosaurs, a day lasted about 23 and a half hours.
Because of this, officials have added 27 leap seconds since 1972 to ensure UTC is aligned with astronomical time. The strategy endured for more than 50 years until scientists spotted a fresh reason to fear technological mayhem. In the past decade, Earth’s rotation has sped up, meaning officials might have needed to add the first negative leap second, equivalent to lopping a second off UTC.
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