As the world turns: what time is it? Credit: © NASA

Officials have announced that an extra second will be added to 2005, to accommodate a slowing down of the Earth's rotation.

The announcement is by no means unprecedented. We have been adding leap seconds to years since the 1970s, but, owing to unpredictable quirks in our planet's rotation, we haven't needed one since 1998. The International Earth Rotation and Reference Systems Service in Paris will sneak the extra time in on 31 December 2005, making the countdown to the new year one second longer than some might expect.

The advent of atomic clocks in the 1950s allowed for extremely accurate measurement of periods of time. Starting at a particular point in 1958, an international array of these clocks has been counting out seconds, the length of which was defined at that point. This representation of time is the standard by which the public sets their watches. But people have been keeping an eye on changes in the length of seconds, as fractions of the Earth's daily rotation, using astronomical measurements.

I might do a very geeky thing and watch how my computer behaves. Marcus Kuhn , University of Cambridge, UK

Over the years, the time expressed by atomic clocks has diverged slightly from that determined by the astronomic approach.

This difference results from changes to the Earth's rotation, due to gravitational forces from celestial bodies such as the Moon and the Sun. These forces are thought to cause heavy matter to shift within the Earth's core, explains Markus Kuhn of the University of Cambridge, UK. When heavy materials move toward the planet's centre, it speeds up its rotation just like an ice skater bringing in her (or his) arms for a swift twirl. Other events, including the large earthquake that preceded the Asian tsunami of December 2004, can also alter the Earth's spin by a tiny amount.

Leaping forward

To smooth out wrinkles between atomic and astronomic time, experts introduced the practice of adding and subtracting leap seconds. Leap years come at regular intervals and correct for the slight difference between the time it takes our planet to travel around the Sun and a full 365 days, but leap seconds are more capricious. Rotation is much less predictable than our orbit around the Sun, says Kuhn.

There was a spate of leap seconds in the 1970s. So far 32 leap seconds have been added to years, says Kuhn, and there will be 33 by the end of 2005

Desktop computers will adjust to the added second by talking to other units on the Internet. Kuhn notes that computers switched off for weeks often have to make the same type of time correction. But some experts believe that highly specialized technologies that involve Global Positioning System tools will lack the capability to cope with such a change. For this reason, a movement has emerged to abolish the practice of adding leap seconds.

Kuhn is not convinced that this would be a good idea. "I believe the problem is exaggerated. We can just develop a standard for how computers adjust to the leap second," he says.

So how will Kuhn spend his extra second: "It's quite possible I might do a very geeky thing and watch how my computer behaves."