ARTICLE
from the
Encyclopædia Britannica
second, fundamental unit of time, now defined in terms of the radiation frequency at which atoms of the element cesium change from one state to another.
The second was formerly defined as 1/86,400 of the mean solar day—i.e., the average period of rotation of the Earth on its axis relative to the Sun. In the mid-20th century this definition became inadequate because of the need for increased precision in timekeeping. In 1956 the second was redefined by the International Committee on Weights and Measures as 1/31,556,925.9747 of the length of the tropical (seasonal) year 1900. In 1967 the 13th General Conference on Weights and Measures provisionally defined the second as 9,192,631,770 cycles of radiation associated with the transition between the two hyperfine levels of the ground state of the cesium-133 atom (see atomic time). The number of cycles of radiation was chosen to make the length of the defined second correspond as closely as possible to that of the now obsolete astronomically determined second of Ephemeris Time (defined as the fraction of the tropical year given above). As the rate of rotation of the Earth constantly changes, it is necessary to occasionally add (or theoretically to subtract) a second during the year to ensure the atomic timescale Coordinated Universal Time (UTC) stays in synchronization with nature. This represents the sole definition of the second in the International System of Units (SI).
Jonathan D. Betts
Aspects of the topic second are discussed in the following places at Britannica.
Assorted References
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history (in time (physics): Time units and calendar divisions)
The familiar subdivision of the day into 24 hours, the hour into 60 minutes, and the minute into 60 seconds dates to the ancient Egyptians. When the increasing accuracy of clocks led to the adoption of the mean solar day, which contained 86,400 seconds, this mean solar second became the basic unit of time. The adoption of the SI second, defined on the basis of atomic phenomena, as the...
definitions
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cesium clock value (in atomic clock (instrument);
In 1967 the 13th General Conference on Weights and Measures redefined the second, the unit of time in the International System of Units, in terms of the cesium standard so as to equal the second of Ephemeris Time. The conference defined the second as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state...
in quantum mechanics (physics): Cesium clock )The cesium clock is exceedingly stable. The frequency of the oscillator remains constant to about one part in 1013. For this reason, the device has been used to redefine the second. This base unit of time in the SI system is defined as equal to 9,192,631,770 cycles of the radiation corresponding to the transition between the levels F = 4,...
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Ephemeris Time value (in Ephemeris Time (chronology))
...the Moon were obtained visually by observations of occultations of stars by the Moon. By the time Ephemeris Time was superseded in 1984, it had served two important purposes: (1) the definition of a second of Ephemeris Time served as the basis for the redefinition in 1967 of the SI second on the atomic time scale, and (2) ET was the reference scale used for comparison with rotational time to...
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International System of Units (in International System of Units (SI) (measurement);
...equals 1,000 grams as defined by the international prototype kilogram of platinum-iridium in the keeping of the International Bureau of Weights and Measures in Sèvres, France; for time, the second, the duration of 9,192,631,770 periods of radiation associated with a specified transition of the cesium-133 atom; for electric current, the ampere, which is the current that, if maintained in...
in measurement system: Time: second )The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition, or change in energy level, of the cesium-133 atom.
Other
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Fact Monster - Science - Second
