Here is Isaac Asimov’s take on the 180[sup]o[/sup] difference and the Fahrenheit temperature scale in general. It’s from his essay The Height of Up. By this account Fahrenheit originally set his high mark at body temperature of 12 but changed it to 96 because his thermometer was capable of greater accuracy than had been the case until then. On this scale freezing water was a little under 32 and boiling water was a little under 212, with a difference of not quite 180[sup]o[/sup]. Fahrenheit possibly liked 180 exactly and since it was so close that that anyway, he changed his reference points to two physical phenomena of water. He set the freezing point at exactly 32 and the boiling point exactly 180 away at 212. This made body temperature 98.6, but that’s not a constant and only approximate anyone so it doesn’t matter.
I can’t find any reference to Fhrenheit setting body temperature at 100.
*"But then, in 1714, a German physicist named Gabriel Daniel Fahrenheit made a major step forward. The liquid that had been used in the early thermometers was either water or alcohol. Water, however, froze and became useless at temperatures that were not very cold, while alcohol boiled and became useless at temperatures that were not very hot. What Fahrenheit did was to substitute mercury. Mercury stayed liquid well below the freezing point of water and well above the boiling point of alcohol. Furthermore, mercury expanded and contracted more uniformly with temperature than did either water or alcohol. Using mercury, Fahrenheit constructed the best thermometers the world had yet seen.
With his mercury thermometer, Fahrenheit was now ready to use Newton’s suggestion[sup]*[/sup]; but in doing so, he made a number of modifications. He didn’t use the freezing point of water for his zero (perhaps because winter temperatures below that point were common enough in Germany and Fahrenheit wanted to avoid the complication of negative temperatures). Instead, he set zero at the very lowest temperature he could get in his laboratory, and that he attained by mixing salt and melting ice. Then he set human body temperature at 12, following Newton, but that didn’t last either. Fahrenheit’s thermometer was so good that a division into twelve degrees was unnecessarily coarse. Fahrenheit could do eight times as well, so he set body temperature at 96.
On this scale, the freezing point of water stood at a little under 32, and the boiling point at a little under 212. It must have struck him as fortunate that the difference between the two should be about 180 degrees, since 180 was a number that could be divided evenly by a large variety of integers including 2, 3, 4, 5, 6, 9, 10, 12, 15, 18, 20, 30, 36, 45, 60 and 90. Therefore, keeping the zero point as was, Fahrenheit set the freezing point of water at exactly 32 and the boiling point at exactly 212. That made body temperature come out (on the average) at 98.6°, which was an uneven value, but this was a minor point.
Thus was born the Fahrenheit scale, which we, in the United States, use for ordinary purposes to this day. We speak of “degrees Fahrenheit” and symbolize it as “O F.” so that the normal body temperature is written 98.6° F."*
- In 1701 Isaac Newton suggested that the temperature scale start with the temperature of melting ice as the zero point and the body temperature at 12.