Wait a Second, Then Sing 'Auld Lang Syne'

THE "Mme. Manners" of timekeeping has decreed a change in the year-end countdown New Year's Eve: "...trois, deux, un, un."

The International Bureau of Weights and Measures in Paris has determined that 1995 needs an extra second. The bonus moment will bring a handful of atomic clocks around the world in synch with time as measured by Earth's rotation. The most accurate of these nuclear Rolexes set the official standard on which all other timekeeping is based.

On a daily basis, astronomical time's accuracy would be the envy of any Swiss watchmaker. But over the very, very long haul, "Earth is not a good clock," says Collier Smith of the National Institute of Standards and Technology's laboratory in Boulder, Colo. "Society would not like to see the sun rise at noon."

Astronomical time is tied to Earth's rotation, which is inexorably slowing. So the astro clock also is slowing. More than 600 million years ago, for example, the day was about 21 hours long. Dinosaurs faced a 23-hour day.

One brake on Earth's pirouette comes from winds blowing across the planet's rough surface. "On a daily basis, the effect is random," says Sanjay Limaye, a scientist at the Space Science and Engineering Center at the University of Wisconsin at Madison. "When the wind is from the west, it speeds the earth. When it is from the east, it slows the earth."

But over the course of a year, the net effect is to slow the earth by a few milliseconds.

Earth's own geophysical processes add to the braking power, says Jeffery Hughes, an astronomy professor at Boston University. The molten material beneath Earth's crust generates friction as it circulates beneath crustal plates. And as volcanoes and earthquakes alter the landscape, they change the distribution of mass in ways that slow the planet, much as a spinning ice skater slows as she extends her arms.

Finally, tidal interactions between the moon and Earth contribute to the slowdown. Those forces are responsible for slowing the moon's rotation rate to one full turn each day.

In principle, Earth could face the same future, researchers say. But before the planet can come close to a grinding halt, it will be incinerated as an aging sun moves into its red-giant stage.

Humanity's ability to accurately gauge astronomical time's slowdown comes as scientists try to devise ever more accurate clocks. The best atomic clocks lose about 1 second every 3 million years - an accuracy about 1 million times greater than using Earth's rotation. Yet, faced with demands for even greater accuracy from scientists and businesses, researchers are trying to improve that figure.

Atomic clockmakers take advantage of quantum physics to build their timepieces. Each kind of atom absorbs or emits radiation at a unique frequency when it moves from one energy state to another.

The clocks used as official time standards base their "ticking" on "signals" emitted from cesium atoms - at 9.192631770 gigahertz, a microwave frequency. Cesium is vaporized, and the atoms are propelled through an airless tube and bombarded by microwave energy at the atoms' frequency. The atoms absorb the energy, and when they release it, detectors pick up the signals. Because cesium's "broadcast channel" doesn't vary, the atoms' signal is used to correct the less-stable signal from the microwave transmitter. This constantly corrected signal is electronically sampled and divided until it yields one "tick," per second.

But, says Mr. Collier, atomic clockmakers still have to cope with inaccuracies that stem from the speed of atoms passing the detectors. NIST researchers are exploring ways to slow the atoms down. One would use a combination of lasers and magnetic traps to reduce the atoms' speed. Another would be to use an "atomic fountain" - shooting atoms up a vertical tube, sampling them at the peak of their ascent, then letting gravity pull them back down.

This year, researchers will have an extra second to work on it.