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New Year's Day Delayed By One Second

Just before the stroke of midnight scientists will delay the start of 2006 by adding a "leap second" to accommodate for changes in the Earth's rotation.

Despite a common belief time it constant, the Earth's rotation rate generally slows down or speeds up throughout the year. Some changes are predictable. The gravitational pull of the moon, for example, is slowing the Earth's rotation by roughly two thousandths of a second for each day per century.

"Think of it as an ice skater doing a spin, said Tom O'Brian, chief of the Time and Frequency Division at the National Institute of Standards and Technology (NIST), which coordinates universal time. "As the ice skater puts her arms out as she's spinning, moving some of her mass farther from the center of her body, she slows down and when she pulls her arms in, she speeds up."

Unpredictable changes in rotation speed comes from the circulation of molten rock inside the Earth, and the climate on the planet's surface, such as the way water is distributed throughout the world and whether there is more snow and ice on the mountains farther away from the Earth's core.

"Extremely accurate time and synchronization is crucial to our modern technological society," O'Brian said. Synchronization of the Internet is an important use of accurate timing, as well as cellular phone networks that are required to be "synchronized better than a millionth of a second daily."

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