"Leap Second" To Be Added On December 31

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oi-Staff

By Staff

Tuesday, December 9, 2008, 13:42

U.S. Naval Observatory is going to add a "leap second" to the world's clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). That is rather than the usual 86,400 seconds in a day, the last day of the year will precisely have 86,401 seconds.

This marks the 24th leap second to be added to UTC, a uniform time-scale kept by atomic clocks around the world, since 1972. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31. It was last added on December 31st 2005.

This corresponds to 6:59:59 pm Eastern Standard Time, 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 much 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 that 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.

While tidal friction is the primary reason for adding these leap seconds, the other factors responsible for the variation in the Earth's spin contribute as well. In fact, negative leap seconds are theoretically possible, although all leap seconds to date have been positive.

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 its Master Clock.

The U.S. Naval Observatory, together with the National Institute of Standards and Technology
(NIST), determines time for the United States.