Because clocks made by humans are more constant than Earth's own rotation, 1990 will have one extra "leap second" to adjust for the difference, officials said Sunday.

"It really boils down to the fact that the rotating Earth itself is not a very good timekeeper," said Donald Sullivan, chief of the Time and Frequency Division of the National Institute of Standards and Technology in Boulder, Colo.The world sets its watches by "atomic clocks," which operate by measuring the vibration of a single atom and are accurate to one-billionth of a second per day. The standard second, which is the basic unit of time, is defined by the vibrations of an atom of the metal cesium.

Earth's rotation, by comparison, is off "by a few tenths of a second per year," Sullivan said. Because time measured by Earth's rotation is not as regular as atomic clocks, the clocks slowly get out of sync with Earth.

"The vibrations of an atom are more regular than the rotations of the Earth," Sullivan said.

Although a second here or there may not sound very important, everything from the timing of traffic lights to the precise navigation necessary for sea and air travel depend on very exact time coordination, Sullivan said.

Atomic clocks originally were adjusted to match the varying rotation of the Earth. But this carried the risk of confusion and error. So in 1972, an international agreement let atomic clocks run independently of Earth and allowed for periodically coordinating the two times with a single adjustment.

The International Earth Rotation Service in Paris monitors the difference in the two times and calls for leap seconds when necessary.

Leap seconds have been added at intervals of from six months to two years. The last leap second was added in December 1989. This year's will be the 16th since 1972.

The leap second will be added to atomic clocks around the world at the same moment on Dec. 31 - 23 hours, 59 minutes and 60 seconds Coordinated Universal Time, the world standard.

That's the equivalent of 6:59 p.m. and 60 seconds EST at the atomic clock at the U.S. Naval Observatory in Washington, and 4:59 p.m. and 60 seconds MST at an atomic clock at the National Institute of Standards and Technology in Boulder.

Humans add a day to the year about every four years - on so-called leap years - because it does not always take exactly 365 days for the Earth to move around the sun.