Definition:Time/Unit/Second

Definition
The second is the SI base unit of time.

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 $0 \ \mathrm K$.

Historical Note
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 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 accuracy is challenging.

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

Linguistic Note
The word second derives from the fact that it was originally specified as a second minute part of an hour.

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 16th century.