Anders Celsius was a Swedish astronomer who invented a thermometer based on the boiling point and freezing point of water. We know a slightly improved version of his thermometer by the Celsius name and by the word centigrade.
The words Celsius and centigrade mean the same thing, but style guides say don’t bother with centigrade anymore, use Celsius.
The original Celsius thermometer had water boiling at 0 degrees and freezing at 100 degrees. Others scientists flipped those numbers for more practical measurements of extreme heat. Each degree of temperature later was called centigrade, from the Latin words centum and gradus, or 100 steps.
This centigrade thermometer was named in honor of Celsius in 1948 at the ninth International Conference of Weights and Measures. Since then, degrees centigrade has diminished in use and degrees Celsius has become the more common term.
Celsius is capitalized just as we capitalize Fahrenheit, named for a German physicist, and Kelvin, named for a British physicist. (Just to make things more complicated, Kelvin is capitalized when you're writing about the scale itself, but lowercase when it's used as the unit. Also, Celsius and Fahrenheit use degrees, but Kelvin doesn’t.)
Style guides agree that Celsius should be used instead of centigrade, but they disagree on how to write the abbreviated form, except that there is no need for a period after C. The Associated Press Stylebook doesn’t use the degree symbol and puts a space between the number and the symbol: 32 C. Scientific Style and Format uses the symbol and puts a space after the numeral: 32 °C. The Chicago Manual of Style and the National Geographic Style Manual say no spaces: 32°C, and that seems to be the prevalent format.
Sources: Online editions of Oxford English Dictionary, Merriam-Webster Unabridged Dictionary, American Heritage Dictionary, Associated Press Stylebook, Chicago Manual of Style, Scientific Style and Format, National Geographic Style Manual, Encyclopedia of World Biography, Wikipedia, and Henry Carrington Bolton’s Evolution of the Thermometer 1592-1743 (1900).
Mark Allen is a freelance copy editor based in Columbus, Ohio. Follow him on Twitter at @EditorMark.
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