Second

Second, basic unit of time measurement in the International System of Units. For many years, until 1956, the second was defined as 1/86,400 of the mean solar day (see Day). Advances in physical science in the years following World War II made necessary a more precise definition, and the second was redefined as 1/31,556,925.9747 of the tropical year (see Year), as standardized at zero hours, minutes, and seconds on December 31, 1899, by international agreement in 1956. This definition was accepted until 1967, by which time the need for a still more precise and unvarying standard of measurement had become apparent. Scientists abandoned the use of larger, changing bases and decided to redefine the second in terms of electromagnetic wavelengths. The second is now established as 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of cesium-133.