# Definition:Time/Unit/Second/Historical Note

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## 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 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 accuracy is challenging.

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

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $60$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $60$ - 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $1$: Tokens, Tallies and Tablets: The first numerals