It seems like everything with our calendar is based around 24hr days and the number of 24hr days to revolve around the sun. But a 24hr day can be broken down to 1,440 minutes and in turn 86,400 seconds. How did we (humans) calibrate the second so that exactly 86,400 would be 1 rotation of the earth to the point where we never need something like a “leap second” like we have with leap years?
In: Planetary Science
It was the Babylonians who gave us the 3,600-second hour and the 360° circle. They liked that number. The earth spins as fast as it does and there’s nothing you can do to change it (or keep it from changing). So the job is to subdivide into repeatable, predictable units (hours, minutes, seconds).
You can do this a few ways. Most directly, you can watch the sun move across the sky. But the sun is big and it hurts to look at. You can also look at stars. Different stars move at different rates, but they each have their own path across the sky that unfolds at a particular speed. These techniques are pretty good for hour-level timekeeping. A sundial can tell you the time within a few minutes (while the sun is out and the gnomon is casting a shadow).
For seconds, you need something more precise. The Babylonians used clepsydra, also called water clocks. They were basins calibrated to drain at a precise rate through a hole in the bottom. When you make a good set of them, you can prove to yourself that they drain 86,400 units in one whole day.
Incidentally, there are leap seconds. The earth’s rotation isn’t quite steady enough that there are a precise, repeatable number of seconds in a decade so we have nudged “official” time around to keep it as consistent as we can with the astronomy.
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