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About that extra 1995 second

As millions counted down to the New Year, astronomers and others who depend on precise time added an extra second to their countdown on December 31.

"The International Earth Rotation Service (IERS) added the `leap second' to account for the difference between atomic time and people's perception of time based on the rotation of the Earth," Professor Thomas Herring said.

The time change can cause all sorts of confusion in time-sensitive equipment that runs around the clock, said Dr. Herring, who is associate professor of earth, atmospheric and planetary sciences. In particular, radio astronomers and others who use precise navigation like the Global Positioning System can make miscalculations.

"Someone depending on GPS could end up 300 kilometers off in that one second if they weren't coordinated with their position on the ground," Professor Herring said.

Official time is kept in about 20 places in the world by using the accurate measurement of the oscillation of a cesium atom. While this measurement remains constant, older measurements of time such as the rotation of the Earth are changing-slowing down-according to Professor Herring. "As a result, every six to18 months we need to add one second to keep time in synchronization. If we didn't make the adjustment, the sun would appear to rise earlier and earlier."

The last time adjustment was made on June 30, 1994 and prior to that on June 30, 1993.

"We've known since the 1800s that the Earth's rotation is slowing because of the loss of frictional energy from the ocean's tides," Professor Herring said. "It slows down at irregular intervals because of fluctuations in motions of the Earth's atmosphere and differences in movement between the fluid core and the solid Earth."

In the United States, the US Naval Observatory is the keeper of time. The December 31 time change occured at 23:59:59 Greenwich Mean Time, thus rendering the accurate countdown to this new year as 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 1.

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