Andrew Yee
December 24th 05, 02:18 PM
NEWS! From the NAVAL OBSERVATORY
Washington, D.C.
Information Contact:
Geoff Chester
USNO Public Affairs Office
(202) 762-1438
Dr. Dennis McCarthy
U.S. Naval Observatory, Washington, DC
(202) 762-1837
For Immediate Release, December 22, 2005
U.S. Naval Observatory to Add Leap Second to Clocks
On December 31, 2005 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. This marks the 23rd
leap second to be added to UTC, a uniform time-scale kept by atomic clocks
around the world. Although you normally don't think about it, for most
conventional uses the "civil" time you use is based on UTC. At the U.S.
Naval Observatory, time is determined by averaging the time signals from
cesium beam atomic clocks and hydrogen masers (the last being an
improvement over the tried and true cesium clocks for short periods of
time).
Man's oldest clock has always been the Earth. We know it's morning when
the Sun rises, noon when the Sun is overhead, and evening when the Sun
sets. The Earth's accuracy as a clock is good to about one thousandth of a
second per day -- more than enough accuracy for most people. However, the
invention of "atomic" clocks, which operate by measuring the resonant
frequency of a given atom (currently Cesium, Hydrogen or Mercury) greatly
increased that accuracy, and has led to the capability at the U.S. Naval
Observatory of measuring time to accuracies exceeding a billionth of a
second per day.
Time measured by the rotation of the Earth is not uniform when compared to
the time kept by atomic clocks. In fact, radio telescopes now observe the
most distant objects in the universe, known as quasars, to determine the
irregularities in the Earth's rotation. This important function is
performed continuously by the Naval Observatory in our Very Long Baseline
Interferometry program. As a result of these small irregularities, the
atomic clocks can get out of sync with the Earth.
In 1972, by international agreement, it was decided to let atomic clocks
run independently of the Earth, keep two separate time-scales, and
coordinate the two. In order to keep the difference between Earth time and
atomic time within nine-tenths of a second as the two scales get out of
sync, leap seconds are added to or removed from the atomic time scale. The
International Earth Rotation and Reference System Service (for which the
U.S. Naval Observatory provides the Rapid Service and Prediction Product
Center) is the organization which monitors the difference in the two time
scales 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 two years. This leap second occurs seven years since
the last one. Leap seconds are added because the Earth's rotation tends to
slow down. If the Earth were to speed up, a leap second would be removed.
The U.S. Naval Observatory is charged with the responsibility for precise
determination and management of time dissemination, and as such provides
the Master Clock for the Department of Defense. USNO, together with the
National Institute of Standards and Technology (NIST), determines time for
the entire nation. Modern electronic navigation and communications systems
depend increasingly on precise time and time interval (PTTI). Examples are
the ground-based LORAN-C navigation system and the satellite-based Global
Positioning System (GPS).
These systems are all based on the travel time of electromagnetic signals:
an accuracy of 10 nanoseconds (ten billionths of a second) corresponds to
a positional accuracy of about three meters or 10 feet. In fast
communications, time synchronization is equally important. All of these
systems are referenced to the U.S. Naval Observatory Master Clock.
The present USNO Master Clock is required by the Department of Defense to
be accurate to better than a billionth of a second per day. It is based on
an ensemble of 60 independently operating cesium-beam atomic clocks and 15
hydrogen maser atomic clocks. These clocks operate in environmentally
controlled vaults to ensure their stability. By automatic inter-comparison
of all clocks every 100 seconds, a time scale can be computed which is not
only reliable but also extremely stable. Its rate does not change by more
than about 100 picoseconds (0.000 000 000 1 seconds) per day from day to
day. On the basis of this computed time scale, a clock reference system is
steered to produce clock signals that serve as the U.S. Naval Observatory
Master Clock. USNO also operates an Alternate Master Clock, consisting of
12 cesium-beam clocks and three hydrogen masers, located at Schriever Air
Force Base in Colorado.
Due to the sheer number of independent clocks maintained by the U.S. Naval
Observatory, it is the largest single contributor to the international
time scale (UTC), which is computed in Paris, France, at the International
Bureau of Weights and Measures. Moreover, its principal role in keeping
track of the change in the "Earth clock" (i.e., Earth rotation) and its
dissemination of this information as the Rapid Service and Predictions
Product Center for the International Earth Rotation and Reference System
Service attests to the fact that globally, as well as nationally, the U.S.
Naval Observatory remains the leader in precise time.
Information concerning the U.S Naval Observatory, its mission, history,
and programs is available from our World Wide Web site at
http://www.usno.navy.mil
Washington, D.C.
Information Contact:
Geoff Chester
USNO Public Affairs Office
(202) 762-1438
Dr. Dennis McCarthy
U.S. Naval Observatory, Washington, DC
(202) 762-1837
For Immediate Release, December 22, 2005
U.S. Naval Observatory to Add Leap Second to Clocks
On December 31, 2005 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. This marks the 23rd
leap second to be added to UTC, a uniform time-scale kept by atomic clocks
around the world. Although you normally don't think about it, for most
conventional uses the "civil" time you use is based on UTC. At the U.S.
Naval Observatory, time is determined by averaging the time signals from
cesium beam atomic clocks and hydrogen masers (the last being an
improvement over the tried and true cesium clocks for short periods of
time).
Man's oldest clock has always been the Earth. We know it's morning when
the Sun rises, noon when the Sun is overhead, and evening when the Sun
sets. The Earth's accuracy as a clock is good to about one thousandth of a
second per day -- more than enough accuracy for most people. However, the
invention of "atomic" clocks, which operate by measuring the resonant
frequency of a given atom (currently Cesium, Hydrogen or Mercury) greatly
increased that accuracy, and has led to the capability at the U.S. Naval
Observatory of measuring time to accuracies exceeding a billionth of a
second per day.
Time measured by the rotation of the Earth is not uniform when compared to
the time kept by atomic clocks. In fact, radio telescopes now observe the
most distant objects in the universe, known as quasars, to determine the
irregularities in the Earth's rotation. This important function is
performed continuously by the Naval Observatory in our Very Long Baseline
Interferometry program. As a result of these small irregularities, the
atomic clocks can get out of sync with the Earth.
In 1972, by international agreement, it was decided to let atomic clocks
run independently of the Earth, keep two separate time-scales, and
coordinate the two. In order to keep the difference between Earth time and
atomic time within nine-tenths of a second as the two scales get out of
sync, leap seconds are added to or removed from the atomic time scale. The
International Earth Rotation and Reference System Service (for which the
U.S. Naval Observatory provides the Rapid Service and Prediction Product
Center) is the organization which monitors the difference in the two time
scales 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 two years. This leap second occurs seven years since
the last one. Leap seconds are added because the Earth's rotation tends to
slow down. If the Earth were to speed up, a leap second would be removed.
The U.S. Naval Observatory is charged with the responsibility for precise
determination and management of time dissemination, and as such provides
the Master Clock for the Department of Defense. USNO, together with the
National Institute of Standards and Technology (NIST), determines time for
the entire nation. Modern electronic navigation and communications systems
depend increasingly on precise time and time interval (PTTI). Examples are
the ground-based LORAN-C navigation system and the satellite-based Global
Positioning System (GPS).
These systems are all based on the travel time of electromagnetic signals:
an accuracy of 10 nanoseconds (ten billionths of a second) corresponds to
a positional accuracy of about three meters or 10 feet. In fast
communications, time synchronization is equally important. All of these
systems are referenced to the U.S. Naval Observatory Master Clock.
The present USNO Master Clock is required by the Department of Defense to
be accurate to better than a billionth of a second per day. It is based on
an ensemble of 60 independently operating cesium-beam atomic clocks and 15
hydrogen maser atomic clocks. These clocks operate in environmentally
controlled vaults to ensure their stability. By automatic inter-comparison
of all clocks every 100 seconds, a time scale can be computed which is not
only reliable but also extremely stable. Its rate does not change by more
than about 100 picoseconds (0.000 000 000 1 seconds) per day from day to
day. On the basis of this computed time scale, a clock reference system is
steered to produce clock signals that serve as the U.S. Naval Observatory
Master Clock. USNO also operates an Alternate Master Clock, consisting of
12 cesium-beam clocks and three hydrogen masers, located at Schriever Air
Force Base in Colorado.
Due to the sheer number of independent clocks maintained by the U.S. Naval
Observatory, it is the largest single contributor to the international
time scale (UTC), which is computed in Paris, France, at the International
Bureau of Weights and Measures. Moreover, its principal role in keeping
track of the change in the "Earth clock" (i.e., Earth rotation) and its
dissemination of this information as the Rapid Service and Predictions
Product Center for the International Earth Rotation and Reference System
Service attests to the fact that globally, as well as nationally, the U.S.
Naval Observatory remains the leader in precise time.
Information concerning the U.S Naval Observatory, its mission, history,
and programs is available from our World Wide Web site at
http://www.usno.navy.mil