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Hey, a ‘leap second’ … oh, you missed it


On Wednesday, Dec. 31, 2008 a “leap second” will be added to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC).  This corresponds to 6:59:59 pm Eastern Standard Time (or, like, 1 p.m. here in Hawaii), when the extra second will be inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, DC.   

Historically, time was based on the mean rotation of the earth relative to celestial bodies and the second was defined in this reference frame.  However, the invention of atomic clocks defined a more precise “atomic time” scale and a second that is independent of the earth’s rotation.  

In 1970, an international agreement established two timescales: one based on the rotation of the earth and one based on atomic time.

The problem is the earth’s rotation is very gradually slowing down, which necessitates the periodic insertion of a “leap second” into the atomic timescale to keep the two within 1 second of each other.  

The International Earth Rotation and Reference Systems Service (IERS) is the organization that monitors the difference in the two timescales and calls for leap seconds to be inserted or removed when necessary.  Since 1972, 23 leap seconds have been added at intervals varying from six months to seven years, with the last being inserted Dec. 31, 2005.   

The U.S. Naval Observatory is charged with the responsibility for the precise determination and dissemination of time for the Department of Defense and maintains the Master Clock. Modern electronic navigation and communications systems depend increasingly on the dissemination of precise time through such mechanisms as the Internet-based Network Time Protocol and the satellite-based Global Positioning System. 

The U.S. Naval Observatory is the largest single contributor to the international time scale (UTC), which is computed in Paris, France, at the International Bureau of Weights and Measures.  However, the U.S. Naval Observatory remains the global leader in precise time due to its principal role in keeping track of changes in the “Earth clock” and its dissemination of this information as the Rapid Service/Prediction Center for the IERS.   

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