Clock-syncing effort buys some time

Leap second: A world standard for coordinating precise timepieces and a tardy Earth will give 2005 one extra tick.

August 08, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Attention, party animals: Hold the corks and confetti. New Year's Day 2006 will be delayed.

The Bureau International des Poids et Mesures in Paris has decreed that one "leap second" will be added to the last minute of this December.

The reprieve for the old year will be brief, but enough to give the Earth a chance to spin a bit longer and catch up with humanity's relentlessly exact atomic clocks.

Without leap seconds every now and then, high noon would eventually slip to tea time, or even midnight by the clock.

For those keeping score, this is the 23rd leap second inserted into the calendar since the international system of Coordinated Universal Time was adopted in 1972.

But it's the first leap second since Dec. 31, 1998. As a result, a growing number of industries that now rely on super-precise timekeeping will have to rejigger their clocks for the first time. Many are not happy about it.

Unpopular option

"People like that really don't like the leap second that much. They have to insert it and tell their end users it has occurred," said Geoff Chester, spokesman for the U.S. Naval Observatory. And that's why the folks in Paris have given them a six-month heads-up.

The Global Positioning System, for example, can't add a second without throwing off navigators who use the satellite system in everything from ocean liners to cell phones.

That's why GPS time, which began Jan. 6, 1980, now runs 13 seconds ahead of clock time. GPS gadgets already adjust so that users see their true local time displayed. Now they'll have to adjust again.

Timekeepers around the world would love to break this cycle. But the only likely alternative is letting the leap seconds pile up for a while. For example, we could wait for the Earth to fall 60 seconds behind our atomic clocks. Then we could add a "leap minute" to set things right after 50 years or so.

"The hope is that 50 years from now, somebody smarter will find a better way to do it," Chester said.

The problem is not new. As our ability to measure time and the Earth's rotation has improved, we have struggled to keep them in step. "We like noon to occur when the sun is highest in the sky," Chester said. "The problem is: The Earth's rotation is not constant."

And that makes it a poor clock.

For centuries, the second was defined as one-86,400th of a solar day. When that proved too unreliable (in 1956), it was redefined more accurately as one-31,556,925.9747th of the time it took the center of gravity between the Earth and the moon to circle the sun.

But even that proved inadequate. In 1972, the international community threw in the astronomical towel and switched to atomic clocks. Now a second is defined as the duration of 9,192,631,770 oscillations of the electrons in cesium 133 atoms. "This is the most precise and stable frequency we can currently routinely maintain," Chester said.

So, timekeeping got better. But the problem of the Earth's rotation did not go away. Thanks to the moon's gravity and tidal effects on the planet's crust and oceans, the Earth's spin is slowing down.

That means today will be longer than this date in 1905.

The problem for timekeepers? The error is cumulative. Every day adds two-1,000ths of a second to the difference between solar time and atomic clock time. That grows to about one second every 500 days.

Left alone, clock time would drift out of sync with the sun. So international timekeepers watch the Earth's rotation closely - and adjust their clocks periodically, as needed.

The Naval Observatory does most of the work. Using radio telescopes around the world, the observatory keeps close watch on 600 very remote objects called quasars, which are "bright" in radio wavelengths. Collectively, they make up something called the International Celestial Reference Frame.

Because they're so distant - 10 billion to 12 billion light-years away - these quasars show no apparent motion across the sky. So they provide fixed points of reference for measuring the Earth's rotation.

Data from the observations are stored on magnetic tape or portable hard drives and flown to Washington. There, the observatory calculates the precise speed of the Earth's rotation - and the wobble in its spin axis. "We can see day-to-day changes ... down to a few millionths of a second per day," Chester said.

By agreement, whenever the Earth's rotation drifts 0.9 seconds out of sync with the atomic clocks, timekeepers add one leap second to restore rough parity. Sometimes they choose the end of June; more often, they pick New Year's Eve.

Until Dec. 31, 1998, leap seconds were added nearly every year. But since then, the Earth's rotation hasn't been slowing as rapidly - and nobody really knows why. Some blame shifts of magma in the Earth's core. Others finger the usual suspect - the global weather phenomenon known as El Nino.

"There are probably a zillion other mechanisms involved we don't understand," Chester said.

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