The evolution of time measurement, part 3: atomic clocks [Recalibration]

Atomic clocks fundamentally altered the way that time is measured and kept. Before atomic clocks, the second was defined by dividing astronomical events, such as the solar day or the tropical year, into smaller parts. This changed in 1967, when the second was redefined as the duration of 9,192,631,770 energy transitions of the cesium atom [1]. The new definition meant that seconds were now measured by counting oscillations of atoms, and minutes and hours were now multiples of the second rather than divisions of the day. Atomic clocks also made it easier to measure time intervals shorter than one second. It is interesting to note that fractional second units, including the microsecond (10^-6 s) and nanosecond (10^-9 s), are based on the decimal system and not on the duodecimal or sexagesimal systems that were used to define minutes and hours.