Today gets an extra second so we don't fall behind the times

BIG BEN sets its pendulum to it, ships' captains swear by it and the speaking clock could not function without it. But despite its worldwide reputation for timeless precision, Greenwich Mean Time is lagging behind the scientific standard.

To make up for this discrepancy, New Year's Eve will last one second longer. Scientists will be inserting a "leap second" to recalibrate our time- keeping to chime with the ruthlessly precise international standard, Co-ordinated Universal Time (abbreviated in all languages to UTC).

Because GMT relies on the position of the Earth vis-a-vis the Sun to measure its accuracy, it is vulnerable to natural variations. The Moon, which creates tidal fluctuations, slows the Earth's spin by pulling at the oceans. The molten metal sloshing around in the Earth's core also holds up its rotation.

Seeking a more consistently accurate measure of when things were happening, scientists developed the atomic clock in the 1950s. It relies on the vibrations of the atoms of the metal caesium for its measure. Nine billion periods of a particular vibration of this atom makes up an atomic clock second. Any variation in these vibrations is so far imperceptible to man-made machinery.

Kristen Lippencott, director of the Royal Observatory at Greenwich, said: "GMT has become a catch-phrase for accurate time, like Hoover is for vacuum cleaners."

In 1972, UTC was officially adopted by the scientific community, but few people are aware of the difference between it and GMT. "Airline pilots will quote times in GMT even if they are measured in UTC," said Dr Lippencott.

Tonight there will be 61 seconds in the last minute of the year and the familiar six pips radio time signal will gain an extra pip before the long pip marking the hour.

There will be no extra bong from Big Ben, however. Its pendulum is only ever accurate to plus or minus one second. Since the clock was first fitted in 1859, engineers have been using old pennies to change the pendulum's centre of gravity and gently slow it when it gets too fast, explained Michael McCann, the keeper of the clock. He will be out celebrating when the clock chimes tonight. But two clock engineers, John Tricki and Brian Tipper, will be monitoring Big Ben in case anything does go wrong.