– How did people ever discover the exact length of a single day? How did they decide on (and keep track of) the exact length of, for example, a single second?
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Its was never discovered. 24 hours in a day is what we go by, not the real length of a day. A year is not 365 days either. Leap years are our joke of a way to say we are still wrong to this day. The real measurement of time cant be understood by the average person, so we will falsely continue to call days and years what they are.
Historically, each town would maintain its own time, as marked by its local solar day. A solar day is the time between two consecutive solar noons, which are when the sun at its highest point in the sky.
A second was originally just a subdivision of the day. One full rotation is subdivided into 360 degrees, each of which are divided into 60 minutes, each of which are further divided into 60 seconds.
Because time isn’t something that can actually be measured (“The objective reference of a clock is another clock” – [The man from earth](https://en.wikipedia.org/wiki/The_Man_from_Earth)) a there is no “exact length” of any unit of time. The SI base unit of time is a second, and all other units are defined as multiples of seconds. But, a second is whatever we say it is. For the last few decades since 1967, the official definition of one second is “the duration of 9.192.631.770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.” What that basically means is that there’s an atom that emits radiation very quickly, but at a very consistent rate, so one second is how long it takes for that large number of individual peices of radiation to be emitted (this is how atomic clocks work).
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