The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as 186400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each (24 × 60 × 60 = 86400).

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second
A pendulum-governed escapement of a clock, ticking every second
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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 frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.[1]

This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks.[2] Because the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time[nb 1] to keep clocks in sync with Earth's rotation.

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