# Definition:Time/Unit/Second/Historical Note

## Historical Note on Second

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-Bīrūnī 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.