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Wait a second!

New Year's 2006 will have to wait. An extra second--called a leap second--will be added to the last day of 2005.

For thousands of years, people have defined time by Earth's rotation on its axis: A complete rotation around this imaginary line marks one day.

But Earth isn't spinning as fast as it used to, says Tom O'Brian, chief of the time and frequency division of the National Institute of Standards and Technology. One slowing factor is the moon's gravity. Its pulling force creates a bulge on the part of Earth's surface that's closest to the moon. As Earth spins, the moon continues to tug on that bulge. The result? A slight slowing of Earth's rotation, and a tiny lengthening of each day. "For example, a day is about one or two thousandths of a second longer now than it was in 1905," O'Brian explains.

Each time those added milliseconds approach a full second, scientists add a leap second to atomic clocks. These precise clocks are used as a standard for the world's time.
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Title Annotation:Earth's rotation
Author:Jakubiak, David
Publication:Science World
Article Type:Brief Article
Geographic Code:1USA
Date:Dec 12, 2005
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