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Can't Stand 2008 For One More Second? Too Bad

Can't wait for 2009? A leap second will be added to the end of Dec. 31 to coordinate clocks with the Earth's rotation, so revelers will have to wait just a bit longer to see this sign light up in Times Square. Bryan Bedder/Getty Images hide caption

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Bryan Bedder/Getty Images

If 2008 is already feeling long, hold on to your hats (or your watches) — it's about to get even longer. The U.S. Naval Observatory Master Clock, the nation's official timekeeper, is adding a leap second to the clock this year at midnight Coordinated Universal Time (UTC), or just before 7 p.m. Eastern.

The extra second realigns precise atomic clocks with a time standard linked to the Earth's rotation. This year's leap second will be the 24th added; the first one was in 1972, according to the U.S. Naval Observatory's Time Service Department. The last time a second was tacked on was at the end of 2005.

The difference in time between atomic clocks, which are based on hydrogen and cesium atoms, and time judged by the Earth's rotation is attributed to several factors, including tidal braking, the gradual slowing of the rotation of the Earth because of friction in the oceans.

Tidal braking increases the length of a day by about 2 milliseconds — or two-thousandths of a second — every century. Because the atomic clocks are set to measure a year as it was in 1900, a clock based on the Earth's rotation currently falls about 2 milliseconds behind the atomic clocks every day. After about 500 days, the Earth is a full second behind.

To reconcile the discrepancy between times, leap seconds are added to the atomic clock to keep the difference less than 0.9 seconds. The clock will tick 11:59:59, 11:59:60, 12:00:00, the Time Service Department says.

In contrast, leap years, which occur once every four years, are a result of the Earth's rotation around the sun taking about 365 days and 6 hours, not a perfect 365.