OK, more time for SKN. If you are traveling with your GPS at midnight,
do you suddenly jump 186,000 miles?
http://www.livescience.com/strangenews/081208-leap-second.html
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.
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