NEWS:Leap Seconds

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Got a second? Scientists argue over time tweaks

By Brian Kladko, Globe Correspondent | November 7, 2005

New Year's Eve revelers won't notice it. But when the clock strikes midnight this Dec. 31, we will gain an extra ''leap " second.

After three decades of adding a leap second every 18 months or so to

compensate for the Earth's slowing rotation, the world has gone for

seven years without one. As the rotations continue to slow there will

be many more leap seconds in our future -- so many that some government

scientists think it's time to abandon them.

That isn't a big deal -- unless you're wedded to the quaint notion that time is based on the rising and setting of the sun.

''The definition of a day is what's at risk here, " said Rob Seaman,

an Arizona astronomer and one of many who supports the use of leap

seconds.

But the chronological corrections are considered a nuisance for

communication and navigation networks, which rely on finely

synchronized components to operate properly. The networks must keep

track of every leap second, and there is always the chance that someone

will miss one. When a leap second was added in 1997, Russia's satellite

navigation system shut down for a day, though its handlers have

insisted that the malfunction was a coincidence, not due to the time

change.

Official time, the standard that we all follow (some of us more

closely than others), is currently an amalgam of two times: solar time,

which is the time it takes for the Earth to complete a full rotation,

and the ultra-precise beats counted out by atomic clocks.

They are different because, contrary to the complaints of today's harried parents, the world is slowing down.

The back-and-forth movement of the tides causes the Earth to lose

energy and turn more slowly -- a trend that's been going on for

millennia. It wasn't until we had atomic clocks that we began to

realize that the Earth's rotation was slowing down.

The long-term forecast is for slower and slower rotations. That

means longer days, and that means the sunrise and sunset won't keep up

with the consistent pace of atomic time.

For three decades, the world's timekeepers have been fudging the

difference with leap seconds. It's not too different from resetting a

wristwatch that runs a bit too fast, and it has happened 22 times since

1972.

What has some US government scientists worried is that the

deceleration of the Earth will require more frequent leap seconds,

increasing the chances for error.

A few decades from now, the addition of a leap second will be an

annual ritual, and a few hundred years from now, they will come as

often as once a month, said Dennis D. McCarthy, who recently retired as

the director of the US Navy's Directorate of Time.

McCarthy fears operators of the world's communication and navigation

systems will be so bothered by adding leap seconds, they will create

their own time systems, ignoring official time.

There is already one prominent example of this -- the US Global

Positioning System, which is currently 13 seconds faster than official

time because it has ignored every leap second since its satellites were

launched.

The consequences could be deadly, because GPS and other satellite

navigation systems will someday supplant radar for monitoring

commercial air traffic, said William Klepczynski, a scientist who

advises the State Department on international satellite navigation

issues.

He imagines this scenario: A Boston-to-London aircraft switches in

mid-flight from the American GPS to Europe's satellite navigation

system, without taking account of whatever time difference exists

between the two. ''It's a little bit hypothetical, " he concedes.

But plausible enough, at least for some US government scientists to

draft a proposal to abolish leap seconds. The proposal caused an uproar

among those who toil in the world's observatories and was shelved this

month. It's unclear whether the plan is dead for good.

Astronomers are strongly attached to leap seconds, because to aim

their telescopes they need to know the orientation of the Earth, as

measured by solar time.

It's not just a matter of compensating for the extra seconds, they

say; some telescopes are controlled by old software that would have to

be painstakingly reprogrammed. In terms of complexity, astronomers

compare it to fixing the ''Y2K bug, " in which mainframe computers

throughout the world were reprogrammed for the new millennium --

except, they say, this will be even tougher.

''You actually have to change the logic of the programs to use

completely different algorithms, " said Seaman, of the WIYN Observatory

in Arizona, which is operated by a consortium of universities. He

estimates that the larger of WIYN's two telescopes would cost about

$500,000 to $1 million to reprogram, ''and money in astronomy is always

an issue. "

Besides, there is more at stake than budgets, astronomers say. There

is the notion of time itself: that a day is as long as one complete

rotation of the Earth.

If leap seconds are abandoned and clocks aren't occasionally reset

to account for the slowing of the planet's rotation, official time will

deviate further and further from sunrise and sunset.

''Noon will no longer have any relation to the sun being overhead, " said Alan Rogers, an astronomer at MIT.

It would be a while before that deviation becomes noticeable to most

people. When it does, 500 to 700 years from now, a ''leap hour " would

be added to make everything right.

But Steven Allen, a researcher at the University of California's

Lick Observatory, believes that lots of small adjustments, in the form

of leap seconds, are a better approach than one big adjustment down the

road.

''If you care about what happens in 600 years, and if you care about

leaving a problem that will grow until your descendants have to deal

with it, that's an issue, " he said.