Added Ticktock of the Clock Restarts Time Debate

By Guy Gugliotta
Washington Post Staff Writer
Monday, December 26, 2005

Time marches on, but Earth is falling behind. The solution again this year is to add a "leap second" as 2005 ticks away, so Earth can catch up with the atomic clocks that have defined time since their unerring accuracy trumped the heavens three decades ago.

This will be the first leap second in seven years, and its arrival will be closely watched by physicists and astronomers enmeshed in a prolonged debate over the future of time in a world increasingly dominated by technology.

Some experts think the leap second should be abolished because the periodic, but random, adjustment of time imposes unreasonable and perhaps dangerous disruptions on precision software applications including cell phones, air traffic control and power grids.

Others, however, argue that it would be expensive to adjust satellites, telescopes and other astronomical systems that are hard-wired for the leap second, and besides, people want their watches to be in sync with the heavens.

Nobody knows how disruptive the leap second really is, but researchers hope to find out soon. "We're going to look at what happens this year," said Naval Research Laboratory physicist Ronald Beard. "If there are no significant problems, the whole issue will go away, but I don't expect that to happen."

Leap seconds are an outgrowth of the post-World War II development of increasingly accurate clocks based on the regular vibration, or "resonance," of atoms as they pass through a magnetic field. In 1958 an atomic second was defined as the time it takes for an atom of cesium 133 to tick through 9,192,631,770 cycles.

At that point atomic time and astronomical time are approximately the same, with the traditional astronomical second defined as 1/86,400th of a "mean solar day," the average time between two consecutive noons.

The trouble is that the heavens behave more capriciously than cesium. Also, the length of Earth's day is increasing by about two milliseconds per century because of the tides, whereas today's atomic clocks, unaffected by cosmic events, tick away with an accuracy within one second for every 20 million years.

Because of this discrepancy, atomic time has been pulling ahead of astronomical time for the past 47 years. To fix this, the International Telecommunication Union in 1972 stipulated that "Coordinated Universal Time," an atomic time used as the world standard, could not diverge from astronomical time by more than 0.9 seconds.

The adjustment tool was the leap second, to be added or subtracted at the discretion of the International Earth Rotation and Reference Systems Service, either at the end of June or the end of the year. Beginning in 1972, there have been 21 leap seconds, the last one in 1998.

"Astronomers wanted a time scale that represented the Earth's movement, and the clock community wanted a smooth scale," said physicist Judah Levine of the National Institute of Standards and Technology and the University of Colorado, who favors eliminating leap seconds. "The compromise has become increasingly difficult to maintain."

The majority of scientists appear to agree that adding leap seconds, up to now an infrequent exercise, is likely to become a twice-a-year experience in about a century as Earth keeps slowing and the unvarying atomic clocks keep ticking away.

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