Pondering the question of why the second is defined as 1/86,400 of a day. I’d always thought it was because a second was about the length of a heartbeat, and thus measurable with the body. (Just like inches and feet and the like.) But that doesn’t really explain the exact number of 86,400 so I did some reading.
- Egyptians used base 12 math for some astronomy related things. Why 12? Either because you can count to 12 with your finger joints or maybe because 12 = 2*2*3. Or maybe because there’s ~12 lunar cycles in a year.
- Egyptians divided daytime into 12 hours and nighttime into 12 hours. Note these hours were of variable length depending on the season.
- Sumerians and Babylonians used base 60 math. Why 60? Not sure, but 60 = 2 * 2 * 3 * 5 which makes it convenient for a lot of math.
- Greeks borrowed the Babylonian math when they did astronomy, so it was natural to divide an hour into 60 minutes and then later to divide a minute into 60 seconds.
- The etymology of “minute” is from “partes minutae primae”; first small division of the hour. Same word as the adjective “minute”, as in tiny.
- The etmology of “second” is from “partes minutae secundae”, the second division of the hour.
- Some languages have a unit of time called the “third”, 1/60 of a second. Polish (tercja) and Turkish (salise).
There’s also a nice StackExchange discussion on the history of decimal time, a sensible 100,000 seconds in a day or what have you. Including this awesome French Revolution clock, my new coveted possession.