The second (symbol: s) is the SI base unit of time. It is often abbreviated sec. in non-SI usage.


The second is the duration of 9 192 631 770 [[Period (physics) [1] of the radiation corresponding to the transition between the two Hyperfine structure [2] of the ground state [3] of the caesium-133 [4] atom [5].


Originally, the second was known as a "second minute", meaning the second minute (i.e. small) division of an hour. The first division was known as a "prime minute" and is equivalent to the minute we know today.


  • 60 seconds = 1 minute [6]
  • 3 600 seconds = 1 hour [7]
  • 86.4 kiloseconds (86 400 seconds) = 1 day [8] (in the SI sense)


See also explanation at [9]

The definition of the second was later refined at the 1997 meeting of the Bureau International des Poids et Mesures [10] to include the statement

This definition refers to a caesium atom at rest at a temperature of 0 K.

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