The Science Behind Saturday's Leap Second

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The ever-so-slight slowing of Earth's rotation means a leap second must be added to our clocks every few years to keep atomic clocks in sync with Earth, shown here in an image taken by the Apollo 8 astronauts in December 1968.
NASA

THE GIST

— Earth has been falling behind the atomic time at a rate of about 2 milliseconds per day.

— This Saturday will mark the 25th time a leap second has been added since the practice started in 1972.

This Saturday, June 30, expect a lengthier day, as an extra leap second will be added to Earthlings' clocks.

What's behind this leap second? The ever-so-slight slowing of Earth's rotation, or the 24-hour spin that brings the sun into our skies every morning.

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Historically, humans based time on the average rotation of the Earth relative to other celestial bodies, with the second defined by this frame of reference. However, the invention of atomic clocks — accurate to about one second in 200 million years — brought about a definition of a second independent of Earth's rotation. Instead, they're based on a consistent signal emitted by electrons changing energy states within an atom.

Earth has been falling behind the atomic time at a rate of about 2 milliseconds per day, currently trailing the atomic time by six-tenths of a second. Every now and then a leap second must be added to atomic clocks (and thus all of our clocks) to keep in sync with Earth's oddball rotation.

This Saturday will mark the 25th time a leap second has been added since the practice started in 1972. The most recent leap second occurred in 2008 on New Year's Eve.

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This added second will give 2012 an extra one day and one second, as 2012 was also a leap year — when a day is added at the end of February to address a discrepancy between our 365-day calendar and the time it takes the Earth to circle the sun.

At 7:59:59 p.m. Eastern Daylight Time (EDT), or 23:59:59 Coordinated Universal Time (UTC), an extra second will be added to the atomic clock at the U.S. Naval Observatory's Master Clock Facility in Washington, D.C.

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