Distance or time for example. I look at my watch and I can see how long 1 second takes. I can look at a ruler and see how long 1 centimeter is. But how do they make rulers and watches? How do you define what a centimeter or a second is without just saying “1/10 of a decimeter” or “1/60 of a minute” or just pointing at another ruler/watch?
I guess time is easier since you can just reference recurring events (like moon phases for example) and then go down in scale from there until you get hours, minutes, seconds. But distance just seems completely arbitrary.
In: 2
Time because of [Ancient Egyptians and Babylonians](https://www.nist.gov/si-redefinition/second-introduction). 12 hours in a day, 12 hours in a night, then base 60 beyond that for minutes and seconds because the Babylonians were really into base 60. Now it’s based on the number of “ticks” recorded at a specific frequency when dealing with energized cesium.
[Meter](https://www.nist.gov/si-redefinition/meter) was originally based on the distance from the equator to the North Pole but more recently has to do with the distance light travels in a vacuum in an incredibly short period of time.
Edit: sorry, I didn’t pay attention to the sub.
Eli5: meter is based on how far light travels in a specific amount of time, seconds are based on a specific measurement recorded from a specific material.
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