When you glance at a weather forecast and see a temperature in degrees Celsius, you are looking at a scale named after a man who never intended it to work the way it does today. The story of the Celsius scale is a tale of reversed logic, posthumous correction, and a surprisingly late official naming — one that did not happen until 204 years after the inventor's death.

Anders Celsius: Astronomer First, Thermometrist Second

Anders Celsius was born in Uppsala, Sweden, in 1701 into a family of academics. His father was an astronomy professor, his grandfathers were a mathematician and an astronomer. Science ran in the blood. Celsius himself became a professor of astronomy at Uppsala University in 1730, and that was his true calling. His serious scientific work centered on two major problems of the era: the nature of the aurora borealis and the precise shape of the Earth.

On the question of Earth's shape, Celsius was a fierce participant in a genuine scientific controversy. Isaac Newton had argued that the Earth bulges at the equator and flattens at the poles — an "oblate spheroid." French scientists, led by the Cassini family, insisted the opposite was true. Celsius joined the French Lapland expedition of 1736–1737, traveling to the Arctic Circle to make precise geodetic measurements. The data supported Newton. The Earth is indeed flattened at the poles. Celsius returned a celebrated man of science.

The thermometer was almost a side project.

The Original Scale: Upside Down by Design

In 1742, Celsius published a paper in the Annals of the Royal Swedish Academy of Sciences proposing a standardized thermometric scale with two fixed points: the boiling point of water and the freezing point. Both were sound choices, reproducible in any laboratory. But his assignment of numbers was the opposite of what we use today.

In Celsius's original scale, 0 represented the boiling point of water and 100 represented the freezing point. Descending numbers meant increasing warmth. To modern eyes this looks like a simple mistake. It was not. Celsius was an astronomer accustomed to working with instruments outdoors, in cold Nordic conditions, observing phenomena that moved from ambient cold toward heat. His frame of reference ran from the chill of the field toward the warmth of a laboratory. The descending orientation also had a practical elegance for him: outdoor temperatures in Uppsala rarely exceeded what we would call 30°C, and on his scale those readings clustered comfortably in the positive range above his 100-degree freezing mark.

Linnaeus Flips It

Celsius died of tuberculosis in 1744 at the age of 42, only two years after proposing his scale. He did not live to see it become the dominant temperature measurement system for most of the world. That transformation began almost immediately after his death, and the man responsible was not a physicist or an instrument maker — he was a botanist.

Carolus Linnaeus, the father of modern biological taxonomy and a colleague of Celsius at Uppsala, found the inverted scale deeply inconvenient for his greenhouse and botanical work. He needed to track whether temperatures were rising or falling in a way that matched intuitive human experience: higher numbers should mean hotter. Linnaeus began using a flipped version — 0 for freezing, 100 for boiling — and the Uppsala instrument maker Daniel Ekström produced thermometers to this corrected specification. The inverted scale spread rapidly through the scientific community and became the de facto standard.

It is a gentle historical irony that the scale bearing Celsius's name is the one he explicitly did not design.

"Centigrade" to "Celsius": A Name Almost Two Centuries Late

For nearly two hundred years after Linnaeus's correction, the scale was not widely called "Celsius" at all. It was known as the centigrade scale — from the Latin centum (hundred) and gradus (step) — simply describing its structure of one hundred equal divisions between the two fixed points. The name was descriptive and practical, but it was also ambiguous: "centigrade" had other uses in French and Spanish as a unit of angular measurement.

It was not until 1948, at the 9th General Conference on Weights and Measures, that the international scientific community formally adopted "degree Celsius" as the official name, honoring the Swedish astronomer. The symbol °C was standardized. By that point, two centuries of science had been conducted in "centigrade."

Fahrenheit, Kelvin, and Where Celsius Fits

To appreciate the Celsius scale's place in history, it helps to compare it with its rivals. Daniel Gabriel Fahrenheit, a German physicist working in Amsterdam, introduced his scale in 1724 — nearly two decades before Celsius. He anchored it to two reproducible points: the freezing temperature of a brine solution (a mixture of water, ice, and ammonium chloride) as 0°F, and an approximation of human body temperature as 96°F. Water freezes at 32°F and boils at 212°F on his scale. Fahrenheit's thermometers were remarkably accurate for the era, and his scale dominated European science for a generation before the centigrade approach took hold.

William Thomson (Lord Kelvin) took temperature measurement to its logical absolute extreme in 1848, defining a scale anchored not to water's behavior but to the theoretical point at which molecular motion ceases — absolute zero, defined as 0 K, equal to −273.15°C. The Kelvin scale uses the same degree size as Celsius, making conversion between the two trivially simple: K = °C + 273.15.

The World Today — and One Remarkable Coincidence

Roughly 95% of the world's population lives in countries that use Celsius as the standard for everyday temperature. The principal holdout is the United States, along with a handful of Caribbean territories, where Fahrenheit remains the cultural norm for weather, cooking, and body temperature.

For those who move between the two systems, one fact is worth memorizing as an anchor point: −40°C equals exactly −40°F. This is the sole crossover point of the two scales, a consequence of their differing zero points and degree sizes. Below −40, Celsius readings are numerically smaller (less negative) than Fahrenheit; above −40, Celsius readings are numerically smaller than Fahrenheit readings in the positive range. It is a small, elegant piece of arithmetic that connects two scales born in different countries, in different centuries, for entirely different reasons.