Page last updated at 02:11 GMT, Wednesday, 31 December 2008

Extra second to keep UK on time

Big Ben
The timekeepers who look after Big Ben will have to adjust its clock

A "leap second" will be added onto official clocks around the world at midnight to account for the Earth's slowing spin on its axis.

London's Big Ben, whose bongs bring in the new year across the UK, will have its Great Clock adjusted.

And the BBC will add an extra "pip" to mark the delayed start to 2009.

Twenty-three leap seconds have been introduced since 1972 to keep GMT - internationally defined time - and the Earth's rhythms in synch.

Revellers all over the UK will have an extra second to celebrate
Peter Whibberley, NPL

Timekeepers at the National Physical Laboratory (NPL) will make sure Britain remains up-to-date by adding a second to the UK's atomic clocks, kept at the NPL's Teddington base in Middlesex.

While atomic time is extremely consistent, the Earth's rotation - the traditional way of measuring time - varies unpredictably.

This is due to factors such as changes in the atmosphere and the molten core.

As a result, the two methods of measuring time slowly drift apart and leap seconds occasionally have to be added or subtracted to the atomic clocks to make sure astronomical and atomic time remain synchronised, and to ensure the Sun remains overhead at noon.

Peter Whibberley, senior research scientist in NPL's Time Group, said atomic clocks like those at the laboratory are now "more than a million times more stable than the Earth's rotation".

"The difference between atomic time and Earth time has now built up to the point where it needs to be corrected, so this New Year's Eve we will experience a rare 61 second minute at the very end of 2008 and revellers all over the UK will have an extra second to celebrate," he said.

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