Second

SI unit of time

The second (symbol: s) is a unit of time derived from the division of the day, first into 24 hours, then into 60 minutes, and lastly into 60 seconds, for a total of 24 × 60 × 60 = 86400 seconds per day. That definition, based on 1⁄86400 of a rotation of the Earth, is still used by the Universal Time 1 (UT1) system. The current and formal definition in the International System of Units (SI) is more precise:The second [...] is defined by taking the fixed numerical value of the caesium (Cs) frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the Cs-133 atom, to be 9192631770 when expressed in the unit hertz, which is equal to s−1.

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