Celsius

Anders Celsius was born in Uppsala in 1701 into a family of Swedish scientists: his father was a professor of astronomy, his grandfather a mathematician, and another grandfather a famous astronomer. Celsius himself became professor of astronomy at Uppsala University in 1730 and spent years traveling Europe visiting observatories. He participated in a French expedition to Lapland in 1736–1737, which confirmed Newton's prediction that Earth is flattened at the poles—a significant confirmation of Newtonian physics. He also observed the aurora borealis and produced one of the first scientific accounts linking it to variations in the Earth's magnetic field.

In 1742, Celsius published his temperature scale in a paper to the Swedish Academy of Sciences. He anchored it to two reproducible physical constants: the boiling point of water at sea-level atmospheric pressure, and the melting point of snow. These two fixed points gave the scale its precision—unlike the Fahrenheit scale, which Celsius criticized for anchoring to the imprecisely reproducible temperature of a salt-ice mixture. His critical insight was reproducibility: any laboratory anywhere in the world could establish these two points and calibrate from them.

The peculiarity is that Celsius's original scale ran the other way: he set 100 as the freezing point of water and 0 as the boiling point. The reversed scale was likely for practical convenience in astronomy—measuring temperature drops below freezing was more common than measuring cooking temperatures. Carolus Linnaeus, the Swedish botanist who also appears in the history of nicotine, is often credited with reversing the scale to its current orientation around 1744, though some accounts attribute the reversal to Celsius's colleague Mårten Strömer. Celsius died of tuberculosis in 1744, the same year the reversal occurred, and never used the modern version.

The scale was known as the 'Swedish thermometer' or 'centigrade' (from Latin for 'hundred steps') for two centuries. In 1948, the International Committee of Weights and Measures officially renamed it 'Celsius' in Anders Celsius's honor, to avoid confusion with the French centigrade system and to standardize scientific naming conventions. Today, approximately 95 percent of the world's population uses the Celsius scale; the United States and a handful of other countries use Fahrenheit. Every weather forecast, every oven setting, every fever measurement in metric countries invokes an 18th-century Uppsala astronomer who built his scale backward.