Why 2008 really will be the longest of years

It won't amount to an extra bong from Big Ben but, thanks to a wobble in the Earth's rotation caused by solar winds, lunar gravity and space dust, the end of 2008 tonight will take just that little bit longer.

While much of the rest of the country will be clearing their throats for "Auld Lang Syne" or puckering up for a kiss, British timekeepers and physicists around the world will be adding an extra second to the countdown to 2009 to ensure that man-made time is kept in perfect alignment with that of the planet.

The so-called "leap second", which will be added to the world's clocks at 23:59:59 GMT, is necessary to correct the second-long gap that has opened up between the time kept by the 200 or so atomic clocks dotted around the globe and the period it takes for the Earth to complete a single rotation on its axis.

Atomic clocks, driven by the movement of caesium atoms, are accurate to within a billionth of a second per day, but the rate at which the planet revolves can be minutely affected by an array of factors including changes in the atmosphere and magnetic storms. The result is that, every now and again, an extra second must be added to, or subtracted from, human time to ensure the two time scales are kept in synch.

"Atomic clocks are now more than a million times more stable than the Earth's rotation," said Peter Whibberley, a senior research scientist at the National Physical Laboratory in west London. "As a result, the two methods of measuring time slowly drift apart and we occasionally have to add or subtract a leap second to the atomic clocks to make sure astronomical and atomic time remain synchronised.

"The difference between atomic 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 and so have an extra second to celebrate."

Today's leap second will be the 24th bonus moment since Co-ordinated Universal Time, the system used to ensure a common time standard around the world, was introduced in 1972. The next is due in 2012 or 2013.