The Celsius system of temperature measurement is based on the freezing and boiling temperatures of water: 0 degrees is the temperature at which water freezes, and 100 degrees is the temperature at which water boils. That makes sense.
So what is the Fahrenheit system about? 0 degrees F doesn’t seem to be significant; nor does 100. Water freezes at 32 and boils at 212. So what’s going on?
Well, it turns out that we don’t really know. However, there are several interesting theories. Here are some of them. Some only explain one point on the scale, but of course he had to fix at least two points to create a meaningful scale.
1. Body temperature as 100 theory. According to this theory, Fahrenheit decided to set 100 degrees at the human body temperature. He measured his own body temperature and called it 100. Unfortunately, it seems that he was having a bit of a fever that day, because when other people used his thermometer, they found that their temperatures were generally closer to 98.6 (or so) on his measurement scale. So our whole system is based on an error.
2. Body temperature as 96 theory. Same idea as above, but he wanted body temperature to be 96. While 100 is nice if you like decimals, 96 is easily divided into fractions involving even numbers--it’s divisible by 48, 24, 12, 4, 3, and 2. According to this theory, Fahrenheit’s own temperature was a little lower than the average when he took it.
3. Making it easy to discuss the weather theory. According to this theory, he wanted the commonly encountered temperatures in Western Europe to be between 0 and 100--easy for the weatherman to talk about without using negative numbers or triple digits. So he measured the temperature in his Denmark hometown for a couple of winters and set 0 as the lowest temperature he encountered. Similarly, he set 100 to the hottest temperature he encountered in Western Europe.
4. Salt solution as 0 theory. If you’ve ever made homemade ice cream, you know that you use a solution of salt, ice, and water to get water cold enough to freeze the ice cream. It works because salt water can be colder than plain water without freezing. According to this theory, Fahrenheit mixed salt, ice, and water and set 0 degrees as the coldest temperature he could get without freezing it.
5. Freemason theory. According to this theory, Fahrenheit was a Freemason. In Freemasonry, there are 32 degrees of enlightenment, and 32 is the highest. Also, the word “degree” may come from Freemasonry.
I don't know which I believe, except that #3 sounds sort of stupid and #5 sounds very weird.