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Friday, May 8, 2009

History Of Time Part - 2

The "Atomic Age" of Time Standards

Scientists had long realized that atoms (and molecules) have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances are inherently stable over time and space. An atom of hydrogen or cesium here today is (so far as we know) exactly like one a million years ago or in another galaxy. Thus atoms constitute a potential "pendulum" with a reproducible rate that can form the basis for more accurate clocks.
The development of radar and extremely high frequency radio communications in the 1930s and 1940s made possible the generation of the kind of electromagnetic waves (microwaves) needed to interact with atoms. Research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule. In 1949, NIST built the first atomic clock, which was based on ammonia. However, its performance wasn't much better than the existing standards, and attention shifted almost immediately to more promising atomic-beam devices based on cesium.

[Laboratory cesium frequency standard]

The first practical cesium atomic frequency standard was built at the National Physical Laboratory in England in 1955, and in collaboration with the U.S. Naval Observatory (USNO), the frequency of the cesium reference was established or measured relative to astronomical time. While NIST was the first to start working on a cesium standard, it wasn't until several years later that NIST completed its first cesium atomic beam device, and soon after a second NIST unit was built for comparison testing. By 1960, cesium standards had been refined enough to be incorporated into the official timekeeping system of NIST. Standards of this sort were also developed at a number of other national standards laboratories, leading to wide acceptance of this new timekeeping technology.
The cesium atom's natural frequency was formally recognized as the new international unit of time in 1967: the second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency, replacing the old second that was defined in terms of the Earth's motions. The second quickly became the physical quantity most accurately measured by scientists. As of January, 2002, NIST's latest primary cesium standard was capable of keeping time to about 30 billionths of a second per year. Called NIST-F1, it is the 8th of a series of cesium clocks built by NIST and NIST's first to operate on the "fountain" principle.
Other kinds of atomic clocks have also been developed for various applications; those based on hydrogen offer exceptional stability, for example, and those based on microwave absorption in rubidium vapor are more compact, lower in cost, and require less power.
Much of modern life has come to depend on precise time. The day is long past when we could get by with a timepiece accurate to the nearest quarter-hour. Transportation, communication, financial transactions, manufacturing, electric power and many other technologies have become dependent on accurate clocks. Scientific research and the demands of modern technology continue to drive our search for ever more accurate clocks. The next generation of time standards is presently under development at NIST, USNO, in France, in Germany, and other laboratories around the world.
As we continue our "Walk Through Time," we will see how agencies such as the National Institute of Standards and Technology, the U.S. Naval Observatory, and the International Bureau of Weights and Measures in Paris assist the world in maintaining a single, uniform time system.

World Time Scales

In the 1840s a railway standard time for all of England, Scotland, and Wales evolved, replacing several "local time" systems. The Royal Observatory in Greenwich began transmitting time telegraphically in 1852 and by 1855 most of Britain used Greenwich time. Greenwich Mean Time (GMT) subsequently evolved as an important and well-recognized time reference for the world.
In 1830, the U.S. Navy established a depot, later to become the U.S. Naval Observatory (USNO), with the initial responsibility to serve as a storage site for marine chronometers and other navigation instruments and to "rate" (calibrate) the chronometers to assure accuracy for their use in celestial navigation. For accurate "rating," the depot had to make regular astronomical observations. It was not until December of 1854 that the Secretary of the Navy officially designated this growing institution as the "United States Naval Observatory and Hydrographic Office." Through all of the ensuing years, the USNO has retained timekeeping as one of its key functions.
With the advent of highly accurate atomic clocks, scientists and technologists recognized the inadequacy of timekeeping based on the motion of the Earth, which fluctuates in rate by a few thousandths of a second a day. The redefinition of the second in 1967 had provided an excellent reference for more accurate measurement of time intervals, but attempts to couple GMT (based on the Earth's motion) and this new definition proved to be highly unsatisfactory. A compromise time scale was eventually devised, and on January 1, 1972, the new Coordinated Universal Time (UTC) became effective internationally.
UTC runs at the rate of the atomic clocks, but when the difference between this atomic time and one based on the Earth approaches one second, a one second adjustment (a "leap second") is made in UTC. NIST's clock systems and other atomic clocks located at the USNO and in more than 25 other countries now contribute data to the international UTC scale coordinated in Paris by the International Bureau of Weights and Measures (BIPM). As atomic timekeeping has grown in importance, the world's standards laboratories have become more involved with the process, and in the United States today, NIST and USNO cooperate to provide official U.S. time for the nation. You can see a clock synchronized to the official U.S. government time provided by NIST and USNO at http://www.time.gov.
[Twelve of the world's time zones]

The World's Time Zones

In the latter part of the nineteenth century, a variety of meridians were used for longitudinal reference by various countries. For a number of reasons, the Greenwich meridian was the most popular of these. At least one factor in this popularity was the reputation for reliability and correctness of the Greenwich Observatory's publications of navigational data. It became clear that shipping would benefit substantially from the establishment of a single "prime" meridian, and the subject was finally resolved in 1884 at a conference held in Washington, where the meridian passing through Greenwich was adopted as the initial or prime meridian for longitude and timekeeping. Given a 24 hour day and 360 degrees of longitude around the earth, it is obvious that the world's 24 time zones have to be 15 degrees wide, on average. The individual zone boundaries are not straight, however, because they have been adjusted for the convenience and desires of local populations. Interestingly, the standard timekeeping system related to this arrangement of time zones was made official in the United States by an Act of Congress in March 1918, some 34 years following the agreement reached at the international conference. In an earlier decision prompted by their own interests and by pressures for a standard timekeeping system from the scientific community — meteorologists, geophysicists and astronomers — the U.S. railroad industry anticipated the international accord when they implemented a "Standard Railway Time System" on November 18, 1883. This Standard Railway Time, adopted by most cities, was the subject of much local controversy for nearly a decade following its inception.

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