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 \u2013 and, worse, 59 seconds \u2013 instead of a round 60.<\/p>\n
\u201cAfter 30 years of debate, the leap second is making way for the leap hour,\u201d said Dr Setnam Shemar, principal scientist in the time and frequency department at the UK\u2019s National Physical Laboratory (NPL). \u201cThis will reduce the risks to the systems and national infrastructure that depend on accurate time while still preserving the fundamental link between our global time system and the rotation of the Earth.\u201d<\/p>\n
The vote at Versailles means that coordinated universal time (UTC) can drift up to an hour from astronomical time before a leap hour is needed, but that is unlikely to happen for thousands of years.<\/p>\n
\u201cWe already have daylight saving where we change by an hour and no one gets bothered about that, apart from losing sleep or getting an extra hour\u2019s sleep,\u201d said Shemar, who sat on the international timekeeping expert committee that proposed the leap hour.<\/p>\n
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.<\/p>\n
Leap seconds are a much more modern invention. In 1955, Louis Essen and Jack Parry at the NPL built the world\u2019s 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.<\/p>\n
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\u2019s rotation is far less reliable. Its spin is affected by the planet\u2019s molten core, the shift of the oceans, global jet streams and other natural forces.<\/p>\n
Computers linked with the atomic clock at the National Physical Laboratory in Teddington, England, and other international atomic clocks. Photograph: David Gee\/Alamy<\/p>\n
Broadly, Earth\u2019s 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.<\/p>\n
The strategy endured for more than 50 years until scientists spotted a fresh reason to fear technological mayhem. In the past decade, Earth\u2019s rotation has sped up, meaning officials might have needed to add the first negative leap second, equivalent to lopping a second off UTC. The 30% probability of a negative leap second in the next decade brought the Versailles vote forward.<\/p>\n
\u201cLeap seconds can be tricky enough to deal with,\u201d said Shemar. Telecommunications, electricity grids and navigation systems used high-accuracy timing, he explained, but software updates could apply leap seconds at the wrong time, knocking networks out of sync.<\/p>\n
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