By Cheryl Pellerin, Washington File Staff Writer
28 December 2005

Washington -- For the first time in seven years, physicists at the U.S. National Institute of Standards and Technology (NIST) in Maryland, and the rest of the world’s atomic timekeepers, will insert a second (known as a leap second) into the internationally monitored time scale on December 31.

The extra second is added to Coordinated Universal Time (UTC) -- the official world time, based on the performance of atomic clocks -- to make it agree with astronomical time to within 0.9 second.

Astronomical time is based on the Earth’s rate of rotation. Because atomic clocks are more stable than the Earth’s rotation rate, which can speed up or slow down due to many factors, leap seconds are needed to keep the two time scales in agreement.

Mechanical clocks and watches use oscillating (cycling) balance wheels, pendulums and tuning forks. The oscillations of atoms or molecules is much more accurate.

Atomic clocks are electronic timekeeping devices controlled by the oscillations of subatomic particles called electrons in the atoms of certain materials, including the metal cesium.

In 1967, the 13th General Conference on Weights and Measures first defined the second in terms of atomic time rather than the motion of Earth.

They defined a second as "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields."

The first atomic clock, invented in 1948, used the vibrations of ammonia molecules rather than cesium. The error between a pair of such clocks -- the difference in indicated time if both were started at the same instant and later compared -- was usually about one second in 3,000 years.

In 1955 the first cesium-beam clock was put into operation at the National Physical Laboratory at Teddington, England. Such a clock will gain or lose less than a second in 3 million years.

The U.S. standard is the NIST-F1 cesium clock, which went into service in 1999 and should not gain or lose a second in 20 million years.

Many nations maintain atomic clocks at standards laboratories, and the time kept by these clocks is averaged to produce a standard called international atomic time.

Very accurate time signals from these standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or satellites, and the signals are used for such things as tracking space vehicles, electronic navigation systems and studying the movements of Earth's crust.

From 1972, when the world went to the current system of atomic timekeeping, until December 31, 1998, 22 seconds have been added to UTC on different occasions to keep it in sync with the Earth's rotation.

The first leap second was added on June 30, 1972. Leap seconds occur at an average rate of slightly less than one per year.

From 1999 until recently, Earth’s rotation and UTC were close enough so they did not require a leap-second adjustment.

In 2005 a leap second will be inserted on December 31 by adding an extra second to atomic clocks at NIST in Colorado and other sites around the world.

The last second of the year would normally be 23:59:59 UTC on December 31, and the first second of the New Year would be 00:00:00 UTC on January 1, 2006.

The leap second will be added at 23:59:59 UTC on December 31, so atomic clocks will read 23:59:60 UTC before changing to all zeros.

A recent proposal to eliminate leap seconds in the future is still under consideration by the international bodies in charge of coordinating world time.

The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State.