Celsius scale

Anders Celsius proposed a temperature scale in 1742 based on two fixed points anyone could reproduce: the temperature at which water freezes and the temperature at which it boils, both at standard atmospheric pressure. The scale's genius was its universality—water behaves the same in Stockholm as in Paris, making calibration independent of any particular thermometer.

Celsius initially set boiling water at 0° and freezing water at 100°—the reverse of modern convention. Within a year of Celsius's death in 1744, the scale was inverted to its current form, with freezing at 0° and boiling at 100°. Who made the change is disputed; Carl Linnaeus and Mårten Strömer both have claims.

The centesimal structure (100 degrees between fixed points) offered mathematical convenience that Fahrenheit's scale lacked. Calculations involving temperature became simpler when the reference points were powers of ten apart. Scientific applications gradually adopted Celsius; everyday usage varied by region.

The adjacent possible required reliable thermometers—Fahrenheit's mercury instruments had established precision standards—and the recognition that universal fixed points would enable international comparison. Earlier scales had used arbitrary references: Fahrenheit's salt-ice mixture at 0°, Newton's body temperature reference. Water's phase transitions were universal and reproducible.

The Kelvin scale, introduced in 1848, simply shifted Celsius by 273.15 degrees to create an absolute temperature scale starting at absolute zero. Every Kelvin equals one Celsius degree; conversions are trivial. This relationship cemented Celsius's role in scientific work.

Today, Celsius is the standard temperature scale for most of the world, with the United States as the conspicuous exception retaining Fahrenheit for everyday use. The metric system's decimal logic made Celsius natural for scientific applications; historical inertia preserved Fahrenheit in countries that had adopted it earlier.

A Swedish astronomer's simple idea—divide water's liquid range into 100 parts—became the world's temperature standard.