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
The original definition was 1/86400 of a mean solar day.
A mean solar day is the average amount of time between two solar noons (when the sun reaches its peak in the sky) throughout the year.
This definition was changed in the 1940s because quartz crystal clocks were far more precise than mechanical clocks. We were then able to measure the second as a fraction of a year because we could keep clocks accurate for that long.
Prior to this, clocks needed to be set regularly, usually at solar noon as measured from the nearest town. After the invention of timezones, it was done from a designated location and transmitted via telegram.
In 1967, we changed the definition once more to be 9,192,631,770 oscillations of a cesium-133 atom. This is incredibly close in duration to the original definition, but most importantly it can be verified independently anywhere in the universe independently of the Earth.
This doesn’t line up perfectly with the old definition, so we occasionally add a leap second to the end of the year (or on June 30th). There have been 27 leap seconds since they were introduced in 1972.
The practice of adding leap seconds his planned to be abandoned by 2035, but a replacement plan has not been implemented yet.
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