SECOND

fundamental unit of time, now defined in terms of radiation emitted from atoms of the element cesium under specified conditions. The second was formerly defined as 1/86,400 of the mean solar dayi.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. This definition was ratified by the General Conference on Weights and Measures in 1960. Because the year 1900 was past, however, the definition was nonoperational. In 1964 the International Committee 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 also atomic time). In 1967 this definition in terms of a property of the cesium atom became the sole definition of the second in the International System of Units (SI). 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 astronomically determined second of Ephemeris Time (defined as the fraction of the tropical year given above).