This a follow up to my earlier question on the lengths of months and weeks.
How did we decide that a second is 1000ms but a minute is 60s and so on
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Before people were comfortable with fractions, they wanted to use numbers that were easily divided. In general these numbers are:
* 12: divisible by 2, 3, 4, and 6.
* 60: divisible by 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30.
* 360: divisible by 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, and 180.
Days, months, and years are objective features of reality and not something we can control (although, conveniently, there *are* approximately 360 days in a year).
The Ancient Mesopotamians divided each day into 12 hours. (60 was considered, but there are very few tasks that take less than a twelfth of a day, and 60 is a large enough number that people would be in danger of losing count of what hour it was.)
Once we got mechanical clocks (as opposed to sundials), the night ended up divided into 12 hours too, for a total of 24 hours between one dawn and the next.
Much later, as science started happening, the Medieval Mesopotamians decided that more precise units of time should be based on the number 60. An hour could be divided into 60 primary-minutes, a primary-minute could be divided into 60 secondary-minutes, a secondary-minute could be divided into 60 tertiary-minutes, and so on.
The minutes and seconds stuck (conveniently, a second is approximately the same length as the interval between human heartbeats), but the thirds weren’t very useful since they were so brief as to require complicated instruments in order to measure; they never saw widespread use. So in modern times when we decided to standardize everything to a single metric unit, we chose seconds as the base unit of time. Things like milliseconds and microseconds aren’t really separate units; “milli” just means “thousandth” and “micro” just means “millionth”.
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