Actually the way I was taught way back in elementary school, Mr. Fahrenheit was very scientific about it. He made himself up a mercury thermometer and one day he went outside and said "It's DAMN cold. I don't think I remember any day as cold as this. I'm going to call this temperature zero." and he marked his tube of mercury.
Then in the summer one day he said, "It's DAMNED hot. I don't remember any day as hot as this. I'm going to call this temperature 100. He marked his tube of mercury.
Then dividing the difference into hundredths, he arrived at the size of a degree in Fahrenheit. That 32º ended up being the freezing temp of water was just an accident.
It was a charmingly human way to reckon temperature and one which divided rough extremes of human tolerance into 100 gradations, a great size for the degree in relation to the human being. That's the essence of imperial and traditional measurements. As the Greeks said, rightly or wrongly, "man is the measure of the universe." According to their way of thinking, only the presence of man makes the universe worthwhile at all. I happen to agree with them that our very existence bears with it a validation of our appropriateness and worth in the universe we inhabit.
Our modern (that should be in quotations, I doubt it is in any way enlightened) way of thinking is that man is inherently evil and the universe would be much better off without us. Tree huggers live their lives with the aim of killing off the vast majority of us and forcing the rest of us to live in the stone age. And they act as if THAT were some kind of a service to the planet, which spawned us as the pinnacle of its accomplishments.
So we adopt systems of measure which have no relationship to man whatever, as man is supposedly irrelevant to the universe he measures. Doesn't the Heisenburg uncertainty principle imply that the act of measurement changes the qualities of that which is measured?
