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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).

Currently, the second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. To allow for the relativistic affects of gravitational time dilation at different altitudes, the second measured for any atomic clock is also corrected to mean sea level.


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