New Year revellers will have to hold their drinks a second longer to usher in year 2009. This is because an extra second will be added to our standard time measurements, which are based on atomic clocks, on the cusp of 2009 to bring it in sync with the rotation of te Earth.

The International Earth Rotation Service, which measures Earth's rotation, will determine the time when the leap happens. The correction is done with a software. Cellphones and computers will automatically reset by picking the new time signals from the internet.

While atomic clocks are accurate, the rotation of the Earth is not. Over a period of time, the time taken by the Earth to rotate around its axis is falling behind by a fraction of a second due to various natural factors such as solar wind, magnetic storms, the atmosphere and even disturbances like earthquakes. This necessitates a correction.

UK's official timekeepers at the National Physical Laboratory consider their atomic clocks more than a million times more stable than the Earth's rotation. "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," says a researcher.

While adding leap seconds to clocks and computer systems is expensive and time consuming, it cannot be avoided. As Peter Whibberley of the National Physical Laboratory notes, if leap seconds were scrapped, over hundreds of years the time of night and day would drift away from the time shown by our clocks. So, one day our clocks would show night happening at midday!

Leap second corrections have come about every 18 months with the last correction done at the end of 2005

Those interested in witnessing the event can visit time.gov just before the update and click on their time zone