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second,abbr. sec or s, fundamental unit of timetime,
sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological.
..... Click the link for more information. in all systems of measurement. In practical terms, the second is 1/60 of a minute, 1/3,600 of an hour, or 1/86,400 of a day. Since the length of the dayday,
period of time for the earth to rotate once on its axis. The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian.
..... Click the link for more information. varies, however, the second must be defined in more precise terms. For many years it was defined as 1/86,400 of the mean solar day (see solar timesolar time,
time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial.
..... Click the link for more information. ), thus eliminating seasonal variations. Because the rotation of the earth itself is not constant, the second was redefined (1956) in terms of ephemeris time (ET), which is calculated from the motions of celestial bodies in accordance with the laws of motion; 1 sec is 1/31,556,925.9747 of the length of the tropical year for 1900. In 1967 the second was redefined to be 9,192,631,770 periods of vibration of the radiation emitted at a specific wavelength by an atom of cesium-133.
secondSymbol: s. The scientific (SI) unit of time, defined since 1967 in terms of atomic time. The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine energy levels of the ground state of the cesium–133 atom.
a unit of measurement of plane angles. It is equal to 1/3600 of a degree or 1/60 of a minute and is denoted by the symbol”.
a unit of time. It is one of the seven base units of the International System of Units. The following definition was adopted by the 13th General Conference on Weights and Measures, held in 1967: the second is 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 cesium-133 atom. The second defined in this manner is known as the atomic second. It can be reproduced by means of cesium frequency and time standards, which permit the determination of the frequency of the radiation of cesium-133 atoms when transitions occur between two fixed energy levels (seeQUANTUM FREQUENCY STANDARDS).
Besides the atomic second, such sciences as astronomy and geodesy make use of the second whose definition is based on the period of revolution of the earth about the sun. This unit is called the ephemeris second, and it is determined from astronomical observations. The ephemeris second is taken as 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time—that is, the year beginning at noon on Dec. 31, 1899. The precise date is indicated in the definition of the ephemeris second because the tropical year is not a constant.
Before the introduction of the ephemeris second in 1956, the time standard was the second defined as 1/86,400 of the mean solar day. This unit, however, was insufficiently stable because of variations in the speed of rotation of the earth. The introduction of the ephemeris second and then of the atomic second permitted the precision of the time standard to be improved by several orders of magnitude. When the atomic second is reproduced by cesium standards, the error at the present time is approximately ± 1 × 10-12.
REFERENCEVremia i chastota. Collection of articles edited by J. L. Jespersen et al. Moscow, 1973. (Translated from English.)
N. S. BLINOV