Based on human perception, based on water chemistry, based on physics.
Zero Fahrenheit is the freezing point of brine (of a certain concentration). That’s water chemistry.
Originally, 90F was based on the average human body temperature, but that later changed to 96F, which just goes to show how arbitrary that scale is.
It’s all arbitrary. Someone just decided to base a scale off of something and that something isn’t fixed from the start. The meter used to be based off the measurement of the earth, but now it’s based off of light.
It’s just some random semi-useful starting point that we all agree on so we’re using the same language.
The meter did not change, science has merely defined it more precisely and reliably over time. It is a measure of length, still one 40 millionth of the circumference of the earth through the poles. Other definitions like the speed of light definition will give you the same result. These newer definitions have reduced uncertainty and added ways to reproduce its length by natural means. But it’s not like the ‘original’ meter was shorter or longer than today’s meter, at least not by any noticeable margin.
Shifting the top end of a temperature scale by over five percent of the scale is a bit more arbitrary than that.
You’ll be shocked to learn that the distance in Kelvin is also adjusted to water “chemistry”, albeit changing the aggregate state seems more physics to me, since no molceules are reacting with each other.
You can’t change the aggregate state of a single molecule, or how do you mean that? Excluding plasma.
you can of a lot of molecules though. and tgat is classically “physics” rather than “chemistry”. Classical chemistry is reactiona between atoms or molecules to form new ones.
If you get deeper into it, the lines between chemistry and physics blur anyways.
Thankfully that has been redefined using the Boltzmann constant, so now anyone in the universe can agree on °C and K without needing to measure any Vienna standard ocean water.
