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Last Updated: Tuesday, 27 September 2005, 21:31 GMT 22:31 UK
Timekeeping proposal sparks row
By Paul Rincon
BBC News science reporter

Clock (BBC)
Leap seconds were first used in 1972
A proposal to scrap leap seconds - small adjustments made to clock time - could create chaos for astronomers and satellite operators, it is claimed.

Every six months, the Paris Observatory tells the world whether to add or subtract a second from atomic clocks.

This synchronises clock time with the solar time used by astronomers.

The US plan to abolish leap seconds would force astronomers to look for new ways to make sure their telescopes are pointed in the right part of the sky.

The Earth isn't a very good timekeeper
Peter Whibberley, NPL
The row highlights the tug of war between two distinct forms of timekeeping: absolute timekeeping, based on atomic frequencies; and everyday timekeeping, based on the rotation of the Earth (solar time).

The latter has a tendency to drift. For this reason, the standard for everyday timekeeping, known as Coordinated Universal Time, or UTC, must be adjusted every so often.

This takes the form of individual seconds being added or subtracted from the length of a day, either a 30 June or a 31 December.

In this way, clock time is kept in step with solar time, which is used to precisely point telescopes and to find satellites.

Problem export

In a statement, the UK's Royal Astronomical Society (RAS) said it "strongly recommends" that the proposal be shelved.

"The present proposal seeks to solve [one] problem by exporting problems to those who use clock time as a measure of mean solar time," it said.

GPS satellite artist's impression, Nasa
GPS considerations may be driving the proposal, some think
"These include astronomers, satellite operators and potentially all who study environmental phenomena related to the rising and setting of the Sun."

If UTC lost its relationship with the Earth's rotation, the error could increase to several seconds within a few years. It would also very quickly make software and possibly hardware used by astronomers obsolete.

Though this would probably require an expensive one-off change, the astronomers would no longer be able to rely on UTC and would most likely have to use a novel server or program to correct for the changes in Earth's rotation.

Drifting apart

"The Earth isn't a very good timekeeper, it tends to slow down over the centuries due to tidal friction," Peter Whibberley, of the UK's National Physical Laboratory (NPL), told the BBC News website.

"When you compare UTC and the Earth's rotation, the two slowly drift apart."

But the RAS points out that the idea of clock time following solar time is also deeply embedded in contemporary technical culture. Researchers estimate that the difference between UTC and Earth time could increase to about an hour within several hundred years.

The US representation to the International Telecommunications Union (ITU) is proposing to abolish leap seconds from December 2007. The plan will be discussed by the ITU at a meeting in Geneva in November 2005.

"This debate should seek a fair solution that serves both needs for time-keeping," said Mike Hapgood, secretary of the RAS.

Dr Hapgood and the RAS believe the debate on leap seconds has, until now, been a closed shop and that a broader public debate is needed.

"There are a lot of skilled people already involved in the debate; we need them to work together to improve current time-keeping for everyone's benefit and not just for one group," he said.

Software issues

The RAS secretary says he believes that software issues with the US global positioning system (GPS) sat-nav network are driving the proposal.

"GPS is not just about satellite positioning, it's also about providing a time signal. My understanding is that some of the software systems used for GPS find it hard to cope with leap seconds. But it is very much specific to the vendor," he told the BBC News website.

Mr Whibberley said he thought more general software considerations were behind the proposal. "A lot of [software] systems need leap seconds to be programmed in manually," he explained.

"It's a problem for telecommunications network operators - who have a large number of atomic clocks dotted around in their network - and for the military," Mr Whibberley added.

There have been 21 leap seconds since 1972 (all additions) and the next is planned at the end of 2005. Notification of one usually comes about six months in advance of it being added.

The UK Department of Trade and Industry (DTI) is co-ordinating its own response to the American proposal.

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12 Jul 01 |  Science/Nature

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