The international unit of the second was first described by the Greco-Egyptian mathematician Claudius Ptolemy in his work "Almagest" around 150 C.E. He defined the second, or second-minute, as one-sixtieth of a minute.
The second was not used in timekeeping, however, until the development in the 17th century of the first mechanical clocks that could track this unit of time. Marin Mersenne, a French mathematician, developed a pendulum clock that could track one second via the pendulum's swing. The development of the atomic clock in the 20th century supplanted these early timekeeping devices, and the second was redefined in 1967 to mean the span of time for a cesium atom to undergo 9,192,631,770 energy transitions.