On November 27, 1701, in the cold Swedish university town of Uppsala, a baby named Anders Celsius was born into a family of professors. Both his grandfathers had been astronomers, his father was a math professor, and Anders was raised on stars, clocks, and equations. By the time he was 29, he was running the Uppsala observatory himself, climbing mountains in Lapland to measure the exact shape of the Earth, and traveling Europe to see how other scientists worked.
In 1742, Celsius proposed a brand-new temperature scale that would make science easier to share. He suggested using two reliable points everyone could agree on: the boiling and freezing points of water. The catch? In his original version, he set 0 for boiling water and 100 for freezing water - upside-down compared to what we use today. After he died young in 1744, fellow scientists, possibly including the famous botanist Carl Linnaeus, flipped the scale around so 0 became freezing and 100 became boiling. The cleaner version stuck.
Today almost every country on Earth uses Celsius for daily weather, cooking, and science. (The United States is one of the few hold-outs, still using Fahrenheit for the weather.) Anders Celsius also helped record the Northern Lights more carefully than anyone before him, suggesting they had something to do with Earth's magnetism - a guess that turned out to be correct. So whenever a thermometer outside reads 0 or 25 or 100, it's a tiny tribute to a Swedish stargazer who wanted a temperature scale that any kid, anywhere, could read at a glance.
