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
Well, for starters we *do* have leap seconds. We add one to the clock every year or so to make sure the clock aligns with the earth’s rotation. The earth’s rotation is not constant, so leap seconds are added irregularly, whenever the International Earth Rotation and Reference Systems Service deems it necessary.
Leap seconds really mess up computer systems though, so the plan is to abandon them sometime in the next decade.
No one really used seconds and very few people even used *minutes* until the 16th century, simply because there both wasn’t accurate timekeeping, and no one really needed to know anything beyond quarter-hours. In the mid-17th century, the first pendulum clock was invented which could fairly-consistently mark seconds, but the exact definition of a second wasn’t marked down until the 1830s, when it was agreed to be 1/86400th of an average solar day. The physics of pendulum swinging (and other methods of consistently marking time) were very well known by that point, so it was just a matter of making a pendulum the right length.
Still, it wasn’t perfectly accurate and so everyone had slightly different seconds. Good enough for basic astronomy and physics, but in the 20th century we needed to be more precise. Fortunately we discovered quartz timekeeping, where you measure the extremely-consistent vibrations of a piece of quartz. So in 1956, the second got redefined to a tiny fraction of a year, and we could finally consistently measure it using quartz vibrations.
Well, kind of consistently. Turns out quartz isn’t perfect, and perfection is really needed for things like radio communication and nuclear physics. So just 11 years later, the second got changed *again* to be based on how fast radioactive caesium decays.
TLDR: It was an ongoing process that took centuries, and is still ongoing with future changes for the calibration of a second planned.
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