By Rebecca Morelle Science reporter, BBC News
What is a leap second? The BBC's Rebecca Morelle went to Greenwich's Royal Observatory to find out
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* The bizarre world of time zones
* UK atomic clock is most accurate
* At the centre of time
Time, as we know it, could soon be in for a radical change.
This week, scientists at the Royal Society are discussing whether we need to come up with a new definition of the world's time scale: Coordinated Universal Time (UTC).
And the main issue up for debate is the leap second - and whether we should abolish it.
The leap second came into existence in 1972. It is added to keep the time-scale based on atomic clocks in phase with the time-scale that is based on the Earth's rotation.
Atomic clock (NPL) Atomic clocks are much better at keeping time than the Earth
The reason for this is that while atomic clocks, which use the vibrations in atoms to count the seconds, are incredibly accurate, the Earth is not such a reliable time-keeper thanks to a slight wobble as it spins on its axis.
Rory McEvoy, curator of horology at the UK's Royal Observatory in Greenwich, explained: "Since the 1920s, it has been known, and previously suspected, that the motion of the Earth is not quite as constant as we'd first thought."
This means that time based on atomic clocks and time based on Earth drift ever further out of phase over time.
So every few years, before the difference has grown to more than 0.9 seconds, an extra second - called the leap second - is added to snap the two back into synch.
"The International Earth Rotation Service monitors the Earth's activity, and they decide when it is appropriate to add a leap second into our time-scale," said Mr McEvoy.
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Leap seconds are a nuisance”
End Quote Felicitas Arias BIPM
But the call to get rid of the leap second is causing a rift within the international time community, and it will come to a head at a vote at the World Radio Conference of the International Telecommunication Union (ITU) in January 2012 in Geneva.
An informal survey by the ITU earlier this year revealed that three countries - the UK, China and Canada, are strongly against changing the current system.
However 13 countries, including the Unites States, France, Italy and Germany, want a new time-scale that does not have leap seconds. But with nearly 200 member states, this still leaves many others that have yet to reveal their position.
The International Bureau of Weights and Measures (BIPM) in Paris is the international standards organisation that is responsible for maintaining the world's time. It thinks that the leap second should go because these one second adjustments are becoming increasingly problematic for systems that need a stable and continuous reference time-scale.
Clockface The leap second is troublesome for applications that require constant time
Dr Felicitas Arias, director of the BIPM's time department and co-organiser of the meeting at the Royal Society, explained: "It is affecting telecommunications, it is problematic for time transfer by the internet (such as the network time protocol, NTP) as well as for financial services.
"Another application that is really very, very affected by the leap second is time synchronisation in Global Navigation Satellite Systems (GNSS).
"GNSS rely on perfect time synchronisation - and leap seconds are a nuisance."
One problem is that because the changes in the Earth's rotation are not regular, leap seconds are also erratic, and only six months' notice is given for each one.
But the countries that are against losing the leap second, including the UK, say the problems are being exaggerated.
Peter Whibberley, senior research scientist in time and frequency at the National Physical Laboratory, in Teddington, said: "When the UK government did a survey of government agencies, they couldn't find anyone who was concerned with leap seconds. So we don't see the evidence presented for the problems caused by leap seconds as being all that serious."
But decoupling civil time from the Earth's rotation could also have longer-term consequences.
Dr Whibberley explained: "[If you lost leap seconds] UTC would drift apart from time based on the Earth's rotation, it would gradually diverge by an increasing amount of time. Something would have to be done to correct the increasing divergence."
Earth Unlinking time from the Earth's rotation has long-term consequences
Over a few decades this would amount to a minute's difference, but over several hundred years this would mean the atomic clock time-scale and the time-scale based on the Earth's rotation would be out by an hour.
In 2004, the idea of swapping leap seconds for a leap hour in a few hundred years' time was proposed. But Dr Whibberley said most scientists agreed that this would be even more problematic.
He explained: "It was dropped quickly. The general feeling was that you could never implement a leap hour as they are much harder to do than the leap seconds, and if you can't cope with leap seconds, it would be much harder to cope with a leap hour."
One possible solution, if the leap second is abolished, could be to tie in any changes with daylight saving changes - even though this would take place in a few centuries' time.
"Countries could just accommodate the divergence by not putting their clocks forward in the spring, so you'd change your time zone by one hour to bring civil time back into line with the Earth's rotation," added Dr Whibberley.
Dr Arias said it was looking increasingly likely that leap seconds may be voted out in January, but that the meeting at the Royal Society could help to thrash out ideas that could offset any problems this loss could cause.
"The point is we can find a compromise, there are possibilities of leaving the time open for synchronisation in the future," she said.
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