From the question "Why is the absolute zero -273.15ºC?" I understood that 1°C is the 100th part of the difference of melting and boiling temperature of water (this is my high school physics, maybe there is a more accurate definition). So we linked the value of 1°C to a physical quantity.
Then we measure another physical quantity (the absolute zero) and it turns out this is exactly 273.15°C less the melting temperature of water. So how come those two quantities have a rational ratio?
If we look at other constants (gravitational, Planck or whatever) they all are irrational, aren't they?
What did I miss? Is it because the melting point of water is not exactly 0°C (maybe something really close to 0 like 0.000565...)?
Answer
it turns out this is exactly 273.15°C less the melting temperature of water.
Actually, "Kelvin" and "degrees Celsius" are defined such that there are 273.16 degrees between absolute zero and the triple point temperature of water. Degrees Celsius are defined as $K - 273.15$.
The freezing point of water is a measured quantity and is not exactly 273.15K nor 0°C and isn't necessarily a rational number.
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