Changes to the world's time scale debated


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

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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.

Split second

Start Quote

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.

Diverging time

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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  • rate this

    Comment number 106.

    I just got up off the floor from laughing.

  • rate this

    Comment number 105.

    Time is a cruel and elusive mistress. There is no absolute measurement of time, only relative ones. We should treat time like we do weight or length, we have one 'yard stick' as it were, time kept by an atomic clock at a given position, we then relate all measurements that need to be super-accurate to this 'yard-clock' . The rest of us just need to know if it's time for tea.

  • rate this

    Comment number 104.

    @99 If you had any scientific understanding you'd know that the atomic clock is the most accurate measurement of time known and not "imaginary" (Time is a man-made concept anyway or do you think the big bang started with Rolex watches?) . But of course, keep trying to be witty with remarks regarding the French, ho ho, check out their flat caps and baguettes!

  • rate this

    Comment number 103.

    What exactly does Coordinated Universal Time (UTC) mean? One time for the whole world? So it would be daylight and dark at different locations but the time would be universal? I'm sorry, am I completely misunderstanding this?

    I think the idea is very attractive, but how would we then differentiate the difference in day-night and degrees thereof world-wide?

  • rate this

    Comment number 102.

    1 Hour ago
    Yawn! And I care?

    Well don't read the article then! It is titled "Changes to the world's time scale debated".... did you think it was about cookery or something??

  • rate this

    Comment number 101.

    Oh dear I've just wasted my extra second typing this.

  • rate this

    Comment number 100.

    If a leap second is added every 'few years', then, with there being 3,600 seconds in an hour - surely it would take about 10,000 years before the two clocks got out of sync. I think that in 10,000 years time we would have worked out how to implement a leap hour.

  • rate this

    Comment number 99.

    If the error between "real" time (ie the universe's time) and the scientists' "imaginary" time continually drifts further and further apart, then surely the scientists' standard second is wrongly defined.

    I have great difficulty imagining an ego that thinks their second is correct and the solar system is wrong.

    It must be something to do with being French.

  • rate this

    Comment number 98.

    @ 1, Kenny.

    Here's another analogy of your amazing argument;

    Just because 2x2 appears to be four, that doesn't mean the answer is four. EH OH DIPSY.

  • rate this

    Comment number 97.

    >Internet communication uses packets that can go any route, arrive in any order ....

    That's partialy true ( for non-connected traffic such as web browsing, email, ftp), but the bearer that carries these packets needs to be synchronised between the 2 routing networks.

  • rate this

    Comment number 96.


    Which is why the sundial is the only really accurate way to tell what time of day it is.

  • rate this

    Comment number 95.

    "As long as the GPS systems are talking to each other using the same time system (which they are), then they maintain their accuracy."

    Not quite. The satellite clocks experience time variations due to speed & gravitational effects. The errors need to be calculated & compensated for using earth-based clocks. Just 1 day without error compensation = positional errors of the order of kms.

  • rate this

    Comment number 94.

    I keep telling my kids, Every second counts.... (when it comes to time wasting and doing nothing at all).

    Now I will have to tell them.. every leap second counts too. ~10-15seconds will get "wasted" throughout their lifetime...

  • rate this

    Comment number 93.

    >Why all the fuss? Who needs seconds.

    When you are trying to synchronise 2 bits of digital communication equipment, accuracy of thime down to a few milliseconds is required, otherwise no phone calls etc.

    Internet communication uses packets that can go any route, arrive in any order and need only to be reassembled in the same order as the sender numbered them.

  • rate this

    Comment number 92.

    The astronomical position of the earth should take precedence. Leap days allow for regular adjustment for seasons. Hence leap seconds should make regular adjustments for the length of day. The actual trend should be assessed and a 'rule' given e.g.: every 8 years a leap second added on 1st Jan at 00:00. As it is then regular the technology should be able to accommodate much more easily.

  • rate this

    Comment number 91.

    Years ago, such accuracy was not altogether relevent, but now accuracy is vital.
    Satelight positioning signals need to be accurate with those on earth, especially to maintain the now massive numbers of planes in our skys, safely, without such accuracy cruise misiles might as well be random bombs thrown from a Roman Catapult.
    Only a complete ignorant idiot would be-little importance of accuracy

  • rate this

    Comment number 90.

    I'd like to spare a second to discuss this, but I really don't have the time.

  • rate this

    Comment number 89.

    A mistake was made when the leap second was originally invented. Clocks should run a hair faster than "real time", not slower, so that leap seconds are deleted (59-second minute) rather than added (61-second minute). It's adding a second that causes so much trouble. Deleting a second would cause far less trouble with computer systems.

  • rate this

    Comment number 88.

    Surely if GPS relies on a precision clock and therefore is confused by adding seconds, it is equally confused by the Earth's wobble. Do they adjust the satellites' orbits to exactly match that wobble? If they do then they must also be up to the task of coping with the altering the time by a second or so. It is only the satellites that need to know, the GPS on the ground just listens to them.

  • rate this

    Comment number 87.

    "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."

    Assuming "few years" averages out at 5 this would mean 15 seconds in the average lifetime, no human would ever notice this therefore... IT DOES NOT MATTER, use atomic time, the end.


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