Countdown to New Year will take extra second as clocks are synchronised

We will have to wait a little longer than usual before popping the champagne corks this coming New Year's Eve - one second, to be exact. The people who keep tabs on how long a second, minute and hour should be are having trouble keeping their own highly-accurate atomic clocks synchronised with the biggest clock of all, the Earth.

On December 31st, a leap second will be added to the world's clocks at 23 hours, 59 minutes and 59 seconds Co-ordinated Universal Time, which matches our own time zone. So if you want to be accurate you will have to add an extra second to your midnight countdown.

We can blame all of this nuisance on the Earth. We know exactly how long a day is, lasting from one sunrise to the next. This in turn is based on how long the Earth takes to spin on its axis. The Earth is no mean timekeeper, good to about one-thousandth of a second per day. However, this is nowhere near as accurate as the atomic clocks, which can split time down to an accuracy exceeding a billionth of a second per day.

Unfortunately for the world's timekeepers, the Earth's rotation is not constant, and over the past few years it has been gradually slowing down. This opens up a gap between Earth time and atomic clock time, a difference which over decades could put the clocks and the Earth hours out of step.

The international UTC time scale is computed in Paris at the International Bureau of Weights and Measures. An agreement in 1972 dictated that world regulators would let their atomic clocks run independently of the Earth, in effect keeping two separate times. However, they also decided that the difference between the two could never exceed nine-tenths of a second. The International Earth Rotation Service monitors the time difference and periodically calls for leap seconds to be added to keep things in step. The last leap second was added 18 months ago and recent additions have been at intervals varying from six months to 2 1/2 years.

Concerns about accuracy are not simply a matter of the scientists getting fussy. Modern satellite navigation and communications systems, including the Global Positioning System and LORAN-C, depend increasingly on precise time. These systems are based on the length of time it takes for a signal to move between a satellite and the Earth. When time is calculated to about 10 billionths of a second, these systems can pinpoint where a person is to an accuracy of about 10 feet.