Is it something we just made up? Or is it something that was discovered?
In: 1
We used to count in base 12, instead of base 10. You see how your fingers (minus your thumbs) are all divided into three sections by your knuckles? They all count up to 12. Count that five times (one for each finger on your other hand), you get 60. 60 seconds in a minute, 60 minutes in an hour, 24 hour days broken into 2 sections of 12, etc.
Our current time system was derived a long time ago by Societies that uses base 12 math. They used certain astronomical events as a basis for time and divided it up. Because it’s base 12, that’s why it’s odd amounts like 60 or 24.
Originally the second was based on nothing more than the divisions of a 24 hour cycle in base 12.
However, more recently (1967) the second is defined as follows:
>The 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
Not very eli5, but it states that we now have true objective measurement for the unit, instead of just a division of man made units.
The current definition of a second is “a very large number times the time it takes for a specific atom to vibrate in a specific way under specified conditions”. The reason for this definition is that that’s the most precise window of time we can measure, in the form of atomic clocks. The exact number in this definition (9,192,631,770 vibrations) was chosen to match up with earlier definitions, which were based on older ways of keeping time.
Atomic clocks are really *really* accurate – the most accurate one [would be off by less than a minute if you used it to time the time since the beginning of the Universe](https://en.wikipedia.org/wiki/Atomic_clock) – so this definition is pretty precise.
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.
We defined a second
A day was chosen to be 24 hours long ago. The first small part (pars minuta prima in latin) of an hour was called the minute and it was decided there should be 60 in an hour. They repeated the splitting to create the second small part (pars minuta secunda) with the same 60 seconds per minute. The latin is how we ended up with the words for minute and second.
Wayyyy after all this was defined relative to a day on Earth we realized it’d be really handy if you could define things without any Earth reference. That’s how we ended up with definitions based off hyperfine transitions of a Caesium-133 atom. You may not have a 24 hour system to measure and then divide by 86,400 to figure out a second, but if you’re off Earth you probably have a way to find a caesium atom and measure things. You did cross space after all.
We decided.
That’s literally it. The concept of time only exists because we defined it. Life, matter, the universe…everything exists without us. But for us to interact with existence and develop ways to move forward, we have to develop language and systems of measurement to better interact.
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