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2006 Postponed by One (Leap) Second

John Roach
for National Geographic News
December 29, 2005
 
For those of you counting the seconds until 2006, add one.

The world's top timekeepers will insert an extra second—or leap second—just before midnight in coordinated universal time (UTC) on New Year's Eve. (That's the same as 6:59:59 p.m. eastern time on December 31.) UTC is determined by atomic clocks and is five hours ahead of eastern time.

Earth's rotation is ever so slightly slowing down, but atomic clocks remain unwaveringly consistent. The extra second will allow Earth to stay in sync with the ultraprecise clocks, which mark time based on the vibration of atoms.

The planet's slowing is mostly due to the friction of tides raised by the gravitational pull of the sun and the moon. In fact, a day is now about two-thousandths of a second longer than it was a couple centuries ago, scientists say.

To keep today's atomic clocks synchronized with time as measured by Earth's rotation, timekeepers insert a leap second whenever the difference between the two clocks exceeds nine-tenths of a second.

The International Earth Rotation and Reference Systems, near Frankfurt, Germany, coordinates the additions of leap seconds.

This year's "extra" second will be the 23rd leap second added since 1972. In that year atomic timekeeping became the world standard. Before then, authorities relied on the movement of stars.

The most recent leap second was added seven years ago, in 1998.

"For the last several years Earth has behaved well," explained Geoff Chester, a spokesperson for the United States Naval Observatory in Washington, D.C. Together with the U.S. National Institute of Standards and Technology, the observatory keeps official time for the country.

But Earth is predicted to slow down "on a nearly exponential scale" in the decades to come as tidal friction increases, Chester says. And this is a conundrum that has sparked debate among the world's clock-watchers.

Abolish Leap Seconds?

As the Earth slows, the addition of leap seconds will be required more frequently to keep the Earth and atomic clocks in sync.

For a world increasingly dominated by technology, this is a potential headache for people who have to reset personal computers, global positioning systems (GPS), mobile phones, and the like every time a leap second is added.

"They don't like leap seconds because they are irregular and they are inserted on a sort of as-needed basis," Chester said. "We try to give a six-month warning so they can make plans."

Although the addition of leap seconds has never proven a problem, some scientists are afraid that a glitch somewhere along the line will lead to a catastrophic error.

For example, an un-updated GPS navigation system could cause an oil tanker to run aground and spoil a pristine coastline. To avoid such a mishap, critics are pushing for the abolition of leap seconds.

On the other hand, if the atomic clocks and Earth's rotation are allowed to drift apart, eventually—as in, thousands of years from now—"high noon" will come hours before the sun crosses directly overhead.

Seeking to avoid such a distorted view of time, leap second proponents argue for continued coordination among the clocks.

"It's like on a small scale abolishing the leap year—the extra day. If you did that, sooner or later the months would get out of sync with the seasons," said Mark Bailey, the director of the Armagh Observatory in Northern Ireland, United Kingdom.

Bailey wants to keep the tradition of leap seconds alive.

Leap seconds may be an inconvenience for timekeepers, he says. But the abolition of leap seconds could come with unintended consequences.

Many telescopes, he notes, are already programmed to account for the occasional addition of leap seconds. If the practice were abolished, astronomers would have to reprogram much of their equipment.

Ongoing Debate

The U.S. Naval Observatory recognizes the pros and cons of leap seconds. But Dennis McCarthy, former head of the observatory's Directorate of Time, opposes the added seconds.

McCarthy is part of a group of U.S. scientists within the UN's International Telecommunication Union (ITU) that recently submitted a proposal to eliminate leap seconds to the ITU, which has final say.

For most timing applications, the group argues, the practice of adding the occasional leap second creates more problems than it fixes.

A working group within the International Astronomical Union, which also has some influence in the matter, is reviewing the proposal. The IAU group will report its findings at the union's generally assembly meeting in August 2006.

The U.K.'s Royal Astronomical Society issued a statement on December 23 calling for a "much wider, more informed public debate" on the usefulness of leap seconds before a final decision is made.

Bailey of the Armagh Observatory, who is also a fellow of the Royal Astronomical Society, added, "It is an interesting question and I think one that deserves wider debate than just among professional scientists."

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