Time experts debate whether to abolish the leap second

What is a leap second? The BBC's Rebecca Morelle went to Greenwich's Royal Observatory to find out

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The future of the world's time is being debated at a meeting in Switzerland.

Experts at the International Telecommunication Union are deciding whether to abolish the leap second.

This is an extra second that is added every few years to keep time measured by atomic clocks in sync with the time based on the Earth's rotation.

Countries such as the United States, France and Germany want to lose the leap second, but the UK, along with China and Canada, wants it to stay.

Bad timing

Start Quote

It would be perhaps the most fundamental change to timekeeping for hundreds of years”

End Quote Peter Whibberley National Physical Laboratory

The proposal to eliminate leap seconds will be discussed on Thursday afternoon at the Radiocommunication Assembly meeting in Geneva.

If agreement amongst the 200 member states cannot be found, the issue will go to a vote.

Ron Beard, chairman of the ITU's working party on the leap second, said: "This is not a technical issue, it is more a diplomatic one."

The world's timescale, Coordinated Universal Time (UTC), is based on the time measured by atomic clocks, which use the incredibly regular vibrations in atoms to count the seconds.

But these clocks are so accurate, they put our former timekeeper - our planet - to shame.

The Earth speeds up and slows down as it spins, which means that while one rotation is one day, some days end up being a few milliseconds longer or shorter than others.

As a result, leap seconds were established in 1972 to keep the time told by atomic clocks and the Earth's time in phase.

Clocks Six months' notice is given for the addition of a leap second

They are added once the International Earth Rotation Service, which monitors the Earth's activity, has found that the two have drifted out of time by 0.9 seconds.

Six months' notice is given for these incremental additions.

But those seeking to abolish the leap second say these one-second jumps are becoming increasingly problematic for navigation and telecommunication systems that require a continuous time reference.

These include satellite navigation, financial services, the internet, flight control and power systems, among others.

Dr Felicitas Arias, director of the time department at the International Bureau of Weights and Measures (BIPM) in Paris, said: "When leap seconds were defined, this was a request from maritime navigation - and today maritime navigation can use other ways to access rotational time.

"So there is no more real need for that synchronisation with a leap second."

However, those who want to keep leap seconds say that the difficulties they cause are not enough to justify abolishing them.

Out of synch

Peter Whibberley, senior research scientist in time and frequency at the National Physical Laboratory, in Teddington, UK, said: "A decision to stop using leap seconds to keep UTC aligned with mean solar time would be perhaps the most fundamental change to timekeeping for hundreds of years.

Artist's impression of Galileo deployment Leap seconds are problematic for satellite navigation systems

"For the first time, civil time worldwide would be based purely on man-made clocks and no longer tied closely to the Earth's rotation."

This could cause some long-term problems.

Over decades, the difference between Earth-based time and atomic clock time would amount to a few minutes, but over 500 years, they would be out by an hour. Over millennia, the discrepancy would grow even more.

The British Science Minister David Willetts said: "The UK position is that we should stick to the current system used throughout the world.

"Without leap seconds we will eventually lose the link between time and people's everyday experience of day and night."

This is not the first time that leap seconds have been brought to the time community's attention.

In 2005, the US proposed that the leap second should be abolished, and replaced with a leap hour, but this failed to be passed by the ITU's members.

This time though, if the decision does come to a vote, a 70% majority will be required in favour of the proposal, for these one second adjustments to go.

If this does happen, the ITU says that leap seconds would be eliminated from 1 January 2018.

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