The Winged Chariot of Time

The Scientific Standard of Time

Until 1955 the scientific standard of time, the second, was based on the Earth's period of rotation and was defined as 1/86,400 of the mean solar day. When it was realized that the Earth's rate of rotation was irregular and also slowing down, it became necessary to redefine the second. In 1955 the International Astronomical Union defined the second as being 1/31,556,925.9747 of the solar year in progress at noon December 31, 1899. The International Committee on Weights and Measures adopted the definition the following year.

With the introduction of atomic clocks—specifically, the construction of a high-precision caesium-beam atomic clock in 1955—more accurate measurement of time became possible. This atomic clock utilizes the frequency of a spectral line produced by the caesium-133 atom. In 1967 the measurement of the second in the International System (SI system) of Units was officially 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.