Second
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The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as ^{1}⁄_{86400} 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 caesium133 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.