YUROP

I didn't know about the arm pit thing.

To create a scale you have to start somewhere.

To think he measured his wife arm pit to be 100°F and then said, let me make a salt brine mix to get to 0°F. Seems completely backwards.

They had no idea about absolute 0 back then. Coldest thing you can create in a lab consistently at the time was a reasonable 0°

But that's where the scale starts and everything is based on.

Could he have created a colder one but wanted 100° to be close to body temperature. Maybe. But that's not weird. 0 and 100 are important numbers. Most people have an armpit and a way to calibrate their thermometer in a pinch. Axillary method, armpit, is a legit way to take someone temperature.

Fahrenheit has 2° compared to 1° of Celsius. 98, 99, 100, 101. (37, 37, 38, 38). Being 1° off in F isn't as much of an error as being 1° off in C.

You can't just boil water and say, that's 100°C! Pressure matters. To boil water at standard pressure was difficult to recreate at the time. Same for freezing.