The New Year Arrived One Second Late
On December 31, 2005 a leap second was added to the world’s clocks at 23 hours, 59 minutes, and 59 seconds Coordinated Universal Time (UTC). This marks the 23rd leap second to be added to UTC, a uniform time-scale kept by atomic clocks around the world.
At the U.S. Naval Observatory (USNO), 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 USNO 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 USNO as part of its 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 to a minimum 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 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.
The leap second at the end of 2005 occured 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 USNO is charged with the responsibility for precise determination and management of time as well as dissemination of that information. USNO, together with the National Institute of Standards and Technology (NIST)(formerly the National Bureau of Standards), determines time for the entire nation.
Modern electronic navigation and communications systems depend increasingly on precise time. 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 -- 10 nanoseconds (ten billionths of a second) of time, for example, corresponds to a positional accuracy of about three meters or 10 feet. In fast communications, time synchronization is
equally important.
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 USNO 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 USNO, 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.
The USNO's principal role in keeping track of the change in the “Earth clock” (i.e., Earth rotation) and its dissemination of this information makes it the world leader in measuring precise time.
For More Information:
http://www.usno.navy.mil/pao/press/2005_Leap_Second.pdf
http://star.arm.ac.uk/press/2005/leap_second2_pr.html
At the U.S. Naval Observatory (USNO), 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 USNO 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 USNO as part of its 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 to a minimum 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 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.
The leap second at the end of 2005 occured 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 USNO is charged with the responsibility for precise determination and management of time as well as dissemination of that information. USNO, together with the National Institute of Standards and Technology (NIST)(formerly the National Bureau of Standards), determines time for the entire nation.
Modern electronic navigation and communications systems depend increasingly on precise time. 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 -- 10 nanoseconds (ten billionths of a second) of time, for example, corresponds to a positional accuracy of about three meters or 10 feet. In fast communications, time synchronization is
equally important.
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 USNO 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 USNO, 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.
The USNO's principal role in keeping track of the change in the “Earth clock” (i.e., Earth rotation) and its dissemination of this information makes it the world leader in measuring precise time.
For More Information:
http://www.usno.navy.mil/pao/press/2005_Leap_Second.pdf
http://star.arm.ac.uk/press/2005/leap_second2_pr.html
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