The Celsius scale, now adopted by almost the entire planet, was born from a Swedish astronomer and went through a twist its own creator never intended.
Anders Celsius was born in 1701 in Uppsala, Sweden, into a family of astronomers and mathematicians — his paternal grandfather, Magnus Celsius, was also a professor of astronomy at the same university where Anders would later teach, and his maternal grandfather was a mathematician. Growing up surrounded by this scientific tradition shaped young Anders early on; he entered Uppsala University still in his teens and became a professor of astronomy there at age 29, a notably young position for the time.
Throughout his career, Celsius traveled extensively across Europe, visiting major observatories in Germany, Italy, and France, and took part in a scientific expedition to Lapland in 1736 that helped resolve one of the great scientific debates of the century: whether the Earth was slightly flattened at the poles or elongated, as predicted by Newtonian physics. The expedition's measurements confirmed the polar flattening, earning Celsius international recognition well before his most famous contribution — the temperature scale.
In 1742, Celsius presented to the Royal Swedish Academy of Sciences a temperature scale based on two fixed points that were easy to reproduce in any laboratory in the world: the freezing and boiling of pure water at sea level. The choice wasn't accidental — Celsius wanted to solve a real practical problem in the science of his time, the lack of standardization between thermometers from different manufacturers and countries, each using its own, often incompatible, reference points.
The best-known historical curiosity about Celsius is that his original scale was inverted compared to the one we use today: he defined 0 degrees as the boiling point of water and 100 degrees as the freezing point — exactly the opposite of today's logic. Historical records suggest this choice had a practical reason: avoiding negative values in typical Swedish weather measurements, since nearly all everyday temperatures would fall between 0 and 100 in that inverted configuration.
It was only after Celsius's death in 1744 — he died young, at 42, likely of tuberculosis — that Swedish botanist Carl Linnaeus (the same one responsible for the biological classification system we still use today) flipped the scale to the format we know: 0 for freezing, 100 for boiling. Some historians also credit this change to other Swedish scientists of the time, such as instrument maker Daniel Ekström, who may have made the inversion independently to simplify thermometer manufacturing.
The scale officially received the name "Celsius" only in 1948, by decision of the General Conference on Weights and Measures — before that, it was commonly called the "centigrade" scale, since it divided the interval between the two reference points into exactly 100 degrees. Today, the Celsius scale is officially defined in terms of the Kelvin scale within the International System of Units, meaning its values are derived from fundamental physical constants, no longer directly from water's freezing and boiling points as in the original definition — though these values remain extremely close in practice.
Celsius never lived to see his scale become the dominant standard it is today: at the height of his career, several competing temperature scales vied for space in Europe, including Fahrenheit (already popular in parts of Europe before becoming nearly exclusive to the United States) and the Réaumur scale, widely used in France and Russia until the 19th century. The Celsius scale's historic victory is partly due to its adoption by the French Metric System and, later, by the International System of Units itself — an example of how institutional standardization decisions can determine, over centuries, which scientific convention survives and which fades away.
Today, the Celsius scale is officially used by virtually every country in the world, with the exception of a small group that maintains Fahrenheit as the predominant cultural standard, mainly the United States. In Brazil, the Celsius scale is universal — it's on every weather forecast, every clinical thermometer, every cooking recipe. It's a curious reminder that a convention born from a practical 18th-century Swedish lab problem would, centuries later, shape how billions of people across the planet describe something as everyday as "it's hot today."