"Leap Second" to be Added
to World Clocks
Written by Nancy
Atkinson
The US Naval Observatory
operates 70 cesium atomic clocks. Credit: USNO
If you ever feel like you need more time, here's some great
news: you're actually going to get it. On December 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, when the extra
second will be inserted at the U.S. Naval Observatory’s Master
Clock Facility in Washington, DC. This is the 24th leap second
added to UTC, a uniform time-scale kept by atomic clocks around
the world, since 1972. Coincidentally, Fraser and Pamela's most
recent episode of Astronomy Cast is about time, so
if you want to know more about time and the atomic clocks used
to provide precise timekeeping, check
it out.
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.
Atomic clocks do not use radioactivity, but they use the exact
frequency of the microwave spectral line emitted by atoms of the
element cesium, in particular its isotope of atomic weight 133
("Cs-133"). The integral of frequency is time, so this
frequency, 9,192,631,770 hertz (Hz = cycles/second), and this
provides the fundamental unit of time, which are measured by
cesium clocks.
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 which
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, with the last being inserted on
December 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 its Master Clock. The U.S. Naval Observatory, together
with the National Institute of Standards and Technology (NIST),
determines time for the United States.
Source: US
Naval Observatory
http://www.universetoday.com/2008/12/09/leap-second-to-be-added-to-world-clocks/
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