Definition:Time/Unit/Second/Historical Note

Historical Note on Second
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 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.