SI second

The second is the fundamental unit of time in the International System of Units (SI). It is currently defined by the frequency of radiation associated with the transition between two specific energy levels of the cesium-133 atom. This definition, established in 1967, equates one second to 9,192,631,770 cycles of this radiation. This precise atomic definition replaced earlier definitions based on the Earth's rotation, which were found to be less accurate over time. Atomic clocks, such as the cesium clock, utilize this atomic vibration to maintain highly accurate timekeeping. Occasionally, leap seconds are added to Coordinated Universal Time (UTC) to keep it synchronized with the Earth's natural rotation.123