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)