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Today we celebrate the birthday of Anders Celsius (27 Nov 1701 – 25 Apr 1744), the Swedish astronomer who gained fame for developing the Celsius temperature scale.

Celcius’ original scale defined 0 °C as the temperature where water freezes, and 100 °C as the temperature where water boils (at one standard atmosphere). This was the definition of the scale until 1954, and remains a useful, pretty accurate approximation, and is still taught in most schools today. However, to be exact, the Celsius scale is currently no longer defined by the freezing and boiling point of water, but rather by the absolute zero temperature and the triple point of purified water. The absolute zero point is defined as -273.15 °C, and the triple point as 0.01 °C.

It’s boiling water, but it sure ain’t 100 °C.
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Based on this slightly redefined scale, the real freezing point of purified water is -0.0001 °C, and its boiling point is 99.9839 °C. Of course these values only apply at exactly one standard atmosphere pressure (approximately sea level) and with specially purified water, so actual ‘real life’ freezing and boiling points only approximate 0 °C and 100 °C anyway. An altitude change of as little as 28 cm causes the boiling point of purified water to change by a thousandth of a degree.

Interestingly, the rule set forth by the International Bureau of Weights and Measures for writing Celsius values (most units of measure, in fact), is to write the numerical value, followed by a space, followed by the °C sign. So the correct way to write a temperature is 37 °C, not 37°C or 37° C.

Currently the Celsius scale is the temperature scale most widely used for all kinds of purposes. Only the United States (bless them) and a handful of other countries still give preference to the Fahrenheit scale. The UK also used to prefer the Fahrenheit scale, but over the last half century the Celsius scale has gained dominance (although they prefer calling it centigrade).

So, whether you prefer an icy, a close to 0°C Scotch on the rocks, or an almost boiling, close to 100 °C cup of coffee or tea, join me in a toast for Anders Celsius, the man who defined it all in the first place.