Redefinition of the Second

At the 13th General Conference on Weights and Measures in October 1967, the SI second was redefined in atomic terms. The new definition established the second as the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the cesium-133 atom's ground state. This replaced the previous definition based on a fraction of the tropical year, transitioning from astronomical to atomic timekeeping. The redefinition provided greater precision and stability for scientific and technological applications, enabling atomic clocks essential for GPS, telecommunications, and scientific research.