With the invention of clocks, finer divisions of time allowed time to be measured in not just days and hours but also minutes and seconds. In Latin, a minute was known as pars minuta prima – ‘the first very small part’ and the second was called the pars minuta Secunda – ‘the second very small part’.
Following the tradition set by the Babylonians, these divisions were expressed using the sexagesimal system, a form of counting based on units of 60. Using this, the length of a second became a sixtieth of a sixtieth of an hour, leading to its definition as 1/3600th of an hour.
One second is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom.