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2008-12-09 08:01:14
Andrea Thompson
Senior Writer
LiveScience.com andrea Thompson
senior Writer
livescience.com Mon Dec 8, 3:24 pm ET
On Dec. 31 this year, your day will be just a second longer.
Like the more well-known time adjustment, the leap year, a "leap second" is
tacked on to clocks every so often to keep them correct.
Earth's trip around the sun - our year with all its seasons - is about 365.2422
days long, which we round to 365 to keep things simpler. But every four years,
we add 0.2422 x 4 days (that's about one day) at the end of the month of
February (extending it from 28 to 29 days) to fix the calendar.
Likewise, a "leap second" is added on to our clocks every so often to keep them
in synch with the somewhat unpredictable nature of our planet's rotation, the
roughly 24-hour whirl that brings the sun into the sky each morning.
Historically, time was based on the mean rotation of the Earth relative to
celestial bodies and the second was defined from this frame of reference. But
the invention of atomic clocks brought about a definition of a second that is
independent of the Earth's rotation and based on a regular signal emitted by
electrons changing energy state within an atom.
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 is very gradually slowing down, continually
throwing the two timescales out of synch, so every so often, a "leap second"
has to be tacked on to the atomic clock.
The International Earth Rotation and Reference Systems Service 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, leap seconds
have been added at intervals varying from six months to seven years - the most
recent was inserted on Dec. 31, 2005.
In the United States, the U.S. Naval Observatory and the National Institute of
Standards and Technology keep time for the country. The Naval Observatory keeps
the Department of Defense's Master Clock, an atomic clock located in
Washington, D.C.
The new extra second will be added on the last day of this year at 23 hours, 59
minutes and 59 seconds Coordinated Universal Time - 6:59:59 pm Eastern Standard
Time.
Mechanisms such as the Internet-based Network Time Protocol and the
satellite-based Global Positioning System (GPS) depend on the accurate time
kept by atomic clocks.