BBC Science Correspondent Sue Nelson: A second is added to synchronise astronomical and atomic clocks

The last minute of 1998 will have 61 seconds. The extra second is being added because the Earth is slowing down.

Our planet is not a perfect sphere. Its slightly squashed shape, combined with the gravitational effects of the Sun and Moon means the Earth's rotational speed is changing.

Daniel Gambis: Earth is like a grapefruit

"The Earth is like a grapefruit," says Daniel Gambis from the International Earth Rotation Service in Paris, which decides if an extra second is necessary.

"So if you consider the attraction of the Sun and the Moon and the bulge of the Earth, this exerts a torque and the effect is to brake the Earth."

Unless modern atomic clocks are adjusted to take account of these slight changes, they would eventually move out of sync with day and night.

John Chambers from the NPL explains why the change is necessary

History of time

Since the dawn of history, our timekeeping has been based on the rotation of the Earth on its axis and its orbit around the Sun. The Babylonians gave us a number system based on the number six and from them our current time divisions have evolved.

The extra second will be added at midnight

The day is divided into 24 hours, each of 60 minutes, each of 60 seconds.

But because the length of the day as determined by the rotation of the Earth varies during the year, it became necessary to define an average day. This explains the name Greenwich Mean Time(GMT).

In the 1950s, the atomic clock was developed. In 1967 the second was defined as the duration of 9 192 631 770 periods of a particular vibration of the caesium-133 atom. This is used to define International Atomic Time (TAI).

The key point is that International Atomic Time is not linked to the Earth's rotation. This means that a clock and calendar based on TAI will become out of step with GMT.

Positive steps

In 1972 Co-ordinated Universal Time (UTC) was adopted. It combines all the regularity of atomic time with most of the convenience of GMT. UTC is kept to within one second of GMT by the addition of extra seconds.

John Laverty: It's all taken care of

A total of 21 leap seconds have been added since 1972. It is possible that at some stage seconds would need to be removed (negative leap seconds). However, all leap seconds so far have been positive.

The new extra second will be inserted in the UK by the National Physical Laboratory in Middlesex - as they also set the time for the telephone speaking clock and the BBC. Those listening closely to the "pips" that mark the hour at midnight on BBC Radio services will hear an extra pip on the signal.

"It all happens electronically," says Dr John Laverty from the National Physical Laboratory (NPL). "All the equipment is programmed well in advance."