How a split second could spell end of GMT

When Big Ben strikes midnight on New Year's Eve, revellers in Trafalgar Square will notice nothing unusual. Keen-eared listeners to the speaking clock, however, will hear an extra pip — the sign of a rare “leap second”. 

The last minute of 2008 will have 61 seconds to correct a slight anomaly between atomic clocks and astronomical time based on the Earth's rotation, so that the proper new year's countdown will end: “three, two, one, one.” 

If an international group of timekeeping experts has its way, however, this year's leap second will be one of the last. The International Telecommunications Union (ITU), which manages leap seconds, is proposing to abolish them in favour of adding a “leap hour” every 600 years or so. 

The move would have important consequences for Britain. Should the leap second be consigned to history, Greenwich Mean Time (GMT) would lose its status as the zone in which local time is the same as the universal time by which clocks are set. Over hundreds of years, this universal time zone would gradually drift east from Greenwich, reaching Paris before the “leap hour” moved it west again.

The proposed change, reported by New Scientist magazine, would also mean that, for the first time, official time would not be linked to the astronomical rotation of the Earth. Instead of seconds, minutes and hours taking their length from the time it takes the globe to spin once, they would be measured solely according to the oscillations of Caesium atoms. 

“It would be a change with profound cultural implications,” said Robert Massey, of the Royal Astronomical Society. “We'd be decoupling our clocks from what the Sun is telling us.” 

It would also have implications for astronomers, requiring expensive changes to the operating software of astronomical telescopes. Sundials would become even more inaccurate than they are already, and it would become almost impossible for sailors to navigate by sextant. 

Britain would have legal issues to grapple with because GMT has been enshrined in law since 1880 as the standard by which national time is calculated. With leap seconds, this is not a problem, as they make GMT and the Universal Co-ordinated Time (UTC) kept by atomic clocks essentially identical. Without them, GMT would have to go. 

The issue has arisen because of the multiple ways in which time is kept. Until 1972 this was done with reference to GMT — the mean solar time at the Royal Observatory in Greenwich. Universal Time, or UT1, is a modern version of GMT, calculated by dividing one rotation of the Earth into 86,400 seconds. The Earth's speed of rotation is, however, very gradually slowing down. Because of this, a new standard was adopted in 1972, based on highly accurate atomic clocks. 

International Atomic Time, or TAI, which is kept by the International Bureau of Weights and Measures in Paris, now defines a second as 9,192,631,770 oscillations of a Caesium-133 atom. This invariable standard is used as the basis for UTC, by which the world's clocks are set. 

Leap seconds are used to keep UTC in line with the variable astronomical timescales, GMT and UT1. Ten leap seconds were added to UTC in 1972, and another 23 have since been added, most recently at the end of 2005. 

The need to do this irritates some scientists who say that leap seconds create confusion and can cause software crashes. A 1996 leap second, for example, caused computers at Associated Press Radio to broadcast the wrong programmes, and a 2003 bug caused some global positioning units to show the time as half past 62. 

The United States proposed a change in 2005 and, though that was defeated, the issue has been raised again this year by the International Telecommunications Union. A working party report published in June found that “a majority supports the suppression of leap seconds”, and will recommend the change. A vote is expected next year and, if 70 per cent of the ITU's 191 members approve, a final decision will be taken at the World Radio Conference in 2011. 

Britain and China oppose the change but it is backed by the US, France, Germany, Russia, Japan and Italy. A spokesman for the Department for Innovation, Universities and Skills said yesterday: “The UK remains strongly opposed to any proposal to cease the addition of leap seconds to UTC.” 

If change does happen, leap seconds will continue to be added as necessary until about 2018. A “leap hour” would then be added, probably around the year 2600. 

Markus Kuhn, a computer scientist at the University of Cambridge, said: “The case for abolishing leap seconds has really not been made, and a leap hour is somewhat unrealistic. 

“Gradually nibbling away a second at a time isn't that disruptive, and there is a distinct lack of scare stories about bad consequences.  continues here

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