Temperature

In 1742, the noted Swedish astronomer Anders Celsius (1701-1744), professor of astronomy at the University of Uppsala, proposed the temperature scale which now bears his name, although for many years it was called the centigrade scale. As with the Fahrenheit scale, the reference points were the normal freezing and normal boiling points of water, but he set them to be 100° apart instead of 180. Because the boiling point and, to a lesser extent, freezing point of a liquid depend on the atmospheric pressure, the pressure must be specified: "normal" means the freezing and boiling points when the atmospheric pressure is exactly one atmosphere. These points are convenient because they are easily attained and highly reproducible. Interestingly, Celsius at first set boiling as zero and freezing as 100, but this was reversed in 1750 by the physicist Martin Strömer, Celsius's successor at Uppsala.

Defined in this way, a Celsius degree (°C) is 1/100 of the temperature difference between the normal boiling and freezing points of water. Because the difference between these two points on the Fahrenheit scale is 180°F, a Celsius degree is 1.8 times (or 9/5) larger than a Fahrenheit degree. You cannot convert between Fahrenheit and Celsius temperatures simply by multiplying by 1.8, however, because their zeroes are at different places. That would be like trying to measure a table in both yards and meters, when the left-hand ends (the zero marks) of the yardstick and meter stick are not starting at the same place.

To convert a temperature from Fahrenheit to Celsius or vice versa, you first have to account for the differences in their zero points. This can be done very simply by adding 40 to the temperature you want to convert. That is because -40° (40 below zero) happens to come out at the same temperature on both scales, so adding 40 gets them both up to a comparable point: zero. Then you can multiply or divide by 9/5 to account for the difference in degree size, and finally remove the 40° that you added.

Thus, to convert from Celsius to Fahrenheit, (1) Multiply by 9/5 (2) Add 32. To convert from Fahrenheit to Celsius, (1) Subtract 32 (2) Multiply by 5/9.