Is it something we just made up? Or is it something that was discovered?
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It was made up by dividing up a day. One day is 24 hours, a hour is 60 minutes and a minute is 60 seconds. So a second is 1/(24*60*60)=86400. That we used the day on earth is easy to understand.
The Babylonians divided 1 degree into 60 minutes and a minute in 60 seconds and that usage has continued to modern times. They had a different name because out is from latin “pars minuta primm” means “first small part” and “pars minuta secunda” is “second small part” . Today we call the usage in angles for arcminute and arcsecond.
The usage of it fro time start with Al-Biruni in 100 CE in regards to Jewish moths. The current usage for start with Roger Bacon in 1267 and started to be common on the clock after Thomas Tompion invented the hairspring 1675.
The length of a day is not as simple as you might imagine because earth’s orbit is elliptical so the time between two solar noon, when the sun is highest in the sky, is not constant. It is 24 hours + 30 second on 21 December an 24 hours -22 seconds on 13 September.
Earth rotational speed is also changing over time, it slows down by 1.78 milliseconds per century, It is not a lot but is need to be handled.
So a second was based on the average length of a day during a year. When we got good enough clocks with quartz crystal oscillators it was defined in 1956 as the average lend of a day in the year 1900. It changed in 1967 to a value based on the transition of caesium-133.
So a second is made up because of the 24 hours, 60 minutes and 60 second is arbitrary. How long a day is on earth is not made up by use but what year we selected for it is.
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