Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
8. Chemistry in the World
What is the difference between Fahrenheit and Celsius?
The two temperature scales were developed by two different people, and they use two different ideas of where to put the zero and 100 marks.
Daniel Fahrenheit based his original temperature scale on three temperatures. The coldest he had available—ice in a brine made from salt and ammonium chloride—would be the zero point (0°). He wanted human body temperature to be near 100°. His third temperature was the melting point of ice. To make it easier to mark his thermometers, he made the freezing point 32° and the human temperature 96°, so he could make the markings by simply cutting the interval between 96° and 32° in half six times.
Later, the scale was adjusted so that water boiled at 212° and froze at 32°. This made human body temperature closer to 98°. The interval between freezing and boiling was 180°, an easy number to divide into parts.
In 1744 a different scale was created, based on one made by Anders Celsius a couple years earlier. His scale used the freezing point of water as 100° and the boiling point of water at sea level as 0°. After his death, the scale was reversed so that higher temperatures had higher numbers.
Because a degree was 1/100 of the difference between melting and boiling, it was called the centigrade scale for over 200 years. But that term was also used for other measurements, and the international standards bureau changed the name to Celsius in 1948.