History of the SI Second

1000 CE, Persian Muslim scholar al-Biruni first used the second: ثانية thaniyah. He defined the second as 1/86400 of a mean solar day (1/ (24*60*60)).


13th century

- Scientists writing in Latin used parte minutae secundae or secunda for short

- Second small part of an hour

- Minute = pars minuta prima “first small part”


1960, the SI second is defined as the ephemeris second: fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time


1967, CIPM changed definition to current definition: duration of 9,192,631,770 periods of the radiation corresponding to the transition of the two hyperfine levels of the ground state cesium-133


1997, CIPM added to definition it refers to a cesium atom at rest at a temperature of 0K


1999, CIPM included corrections for ambient radiation