Swedish astronomer, physicist, and mathematician who introduced the Celsius scale of temperature. His other scientifc works include a paper on accurately determining the shape and size of the Earth, some of the first attempts to gauge the magnitude of the stars in the constellation Aries, and a study of the falling water level of the Baltic Sea.
Celsius was born in Uppsala, where he succeeded his father as professor of astronomy in 1730. He travelled extensively in Europe, visiting astronomers and observatories in particular. In Uppsala he built Sweden's first observatory.
On his travels Celsius observed the aurora borealis; he published some of the first scientific documents on the phenomenon 1733. An expedition to Lapland confirmed the theory propounded by Isaac Newton that the Earth is flattened at the poles.
In 1742 Celsius presented a proposal to the Swedish Academy of Sciences that all scientific measurements of temperature should be made on a fixed scale based on two invariable (generally speaking) and naturally occurring points. His scale defined 0° as the temperature at which water boils, and 100° as that at which water freezes. This scale, in an inverted form devised eight years later by his pupil Martin Strömer, has since been used in almost all scientific work.