Everyone knows how to measure, for example, 1cm or 1 kg of something, but how did people decide how much length is going to be 1 cm or how much weight is going to be 1kg? And how did they make most of the world accept those measurements? Questions applies to other measurements aswell, these two were just examples.
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When the SI units were initially adopted, there was an attempt to have them be based on easily reproducible things. The distance measurement was the “base” of the system – the intention was to have the distance between the Equator and the North Pole be exactly 10,000 km. When that distance was measured, a metal bar representing 1 metre was created. There was some slight error in the measurement that created the definition of the metre, which is why the distance between the Equator and the North Pole is actually more like 10,002 km.
Once the metre was defined, other measurements were defined off of it. The kilogram, for example, is the mass of water in a container that is 1 metre by 1 metre by 1 metre.
Nowadays all of the SI base units have been redefined off of fundamental constants of the universe. So the metre is no longer based off of a survey on the Earth, instead it’s based off of how far light travels in a vacuum in a very specific amount of time.
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