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13 - Definition and Role of a Second

Published online by Cambridge University Press:  01 October 2018

Dennis D. McCarthy
United States Naval Observatory
P. Kenneth Seidelmann
University of Virginia
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Historically the second was considered a 60th of a minute, which was one 60th of an hour. A second based on the Earth’s variable rotation was impractical for modern timekeeping requirements. With the introduction of Ephemeris Time, the ephemeris second was defined in 1954 and revised in 1956 as 1/31 556 925.9747 of the length of the tropical year for 1900.0. The availability of atomic clocks made a more accurate and available second possible. So in 1968, the Système International (SI) second was defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the Caesium 133 atom. With such an accurate measure of the second, the meter was defined as the length of path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. Other SI units are defined in terms of the second and meter. With improved accuracies of timekeeping based on optical frequency standards, a redefinition of the second is under consideration.
Publisher: Cambridge University Press
Print publication year: 2018

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