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The second is the SI base unit of time, and also therefore of the MKS system.

It is also the base unit of time for the FPS and CGS systems.

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 at rest at $0 \ \mathrm K$.


The symbol for the second is $\mathrm s$.

Historical Note

The division of the hour into $60$ minutes, and the minute into $60$ seconds, is a relic of the Babylonian number system, which was a sexagesimal (base $60$) system used mainly for astronomical purposes.

Up until $1967$, the second was defined as being $\dfrac 1 {60 \times 60 \times 24}$ the length of a (solar) day.

This appears first to have been used by al-Biruni in the year $1000$ CE.

It had been noted by astronomical observations that the actual (solar) day is gradually lengthening.

In $1960$, therefore, the second was redefined as:

the fraction $\dfrac 1 {31, 556, 925.9747}$ of the tropical year for $1900$ January $0$ at $12$ hours ephemeris time.

However, even the tropical year is not completely unchangeable, and measuring its duration with high accuracy is challenging.

So in $1967$ the definition of the second was changed again, to what it is now.

Linguistic Note

The word second, in the context of the measurement of time and of angle, derives from the fact that it was originally specified as a second minute part of an hour or of a degree of angle.

The minute had already been defined as a minute part of an hour.

The use of the word second in English, in this specific context, started near the end of the $16$th century.