Anders Celsius built his temperature scale upside down, with boiling at zero
In 1742 the Swedish astronomer Anders Celsius proposed a thermometer scale on which water boiled at 0 and froze at 100. Within two years others had flipped it, and today water's freezing and boiling points are not even part of the definition; ice actually melts a hair below zero.
Celsius's real achievement was careful experiment. He showed that the melting point of ice barely changes with pressure and measured precisely how the boiling point shifts as air pressure varies, proposing that his scale be calibrated at average sea-level pressure. The inverted numbering did not last. In 1743 Jean-Pierre Christin in Lyon published a mercury thermometer running from freezing at 0 to boiling at 100, and in 1744, the year Celsius died, Carl Linnaeus ordered a thermometer the same way round for his greenhouses. His letter of December 1745 about Uppsala's orangery is the first known Swedish document to use the modern direction.
For two centuries the scale was mostly called centigrade, from Latin words for a hundred steps. That caused trouble, because in French and Spanish the same word could mean a fraction of an angle. In 1948 the international weights and measures conference settled on degree Celsius. Adoption in daily life was uneven: Australian weather reports went fully Celsius after 1 September 1972, while BBC forecasts in Britain kept saying centigrade until February 1985.
Meanwhile the definition itself drifted away from water. It moved to the triple point of water, then to water of a precisely specified isotopic make-up, and in 2019 to the Boltzmann constant, so that the kelvin, and Celsius with it, no longer depends on any substance. Absolute zero now sits at exactly −273.15 °C, and each step changed the familiar numbers by less than instruments could detect.
One quirk remains. Because zero is arbitrary, the scale measures intervals, not ratios: something at 20 °C does not hold twice the energy it had at 10 °C. Since 1967, degree Celsius has also been the only SI unit whose full name carries a capital letter.
Source: Celsius
