# Definition:Second of Time/Historical Note

## Historical Note on Second of Time

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 $1960$, the **second** was defined as being $\dfrac 1 {86 \, 400}$, that is $\dfrac 1 {60 \times 60 \times 24}$, the length of a mean 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.

## Sources

- 1964: Milton Abramowitz and Irene A. Stegun:
*Handbook of Mathematical Functions*... (previous) ... (next): $2$. Physical Constants and Conversion Factors - 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $60$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $60$ - 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**second**:**2.** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**second**:**2.** - 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $1$: Tokens, Tallies and Tablets: The first numerals - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next):**second**(time)