atomic second
Quick Summary
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 a transition between two specific energy states of the cesium-133 atom. Specifically, one second is equal to 9,192,631,770 cycles of this radiation.12
This definition, adopted in 1967, replaced earlier definitions based on Earth's rotation, such as 1/86,400 of a mean solar day. The atomic definition provides much greater precision, essential for modern timekeeping and scientific applications. Discrepancies between this atomic timescale and astronomical time, measured by Earth's rotation, are managed by adding occasional leap seconds to Coordinated Universal Time (UTC).134
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