**Anders Celsius** was a Swedish astronomer, physicist and mathematician. He participated in Maupertuis' expedition to measure a degree of latitude in Lapland. He is best known for the temperature scale that he proposed and which is named after him.

- Only when Elvius died in 1718 did Nils Celsius become professor of astronomy.
- After graduating from secondary school, Celsius studied astronomy, mathematics, and experimental physics at the University of Uppsala and gained a deep appreciation for mathematics mainly thanks to Anders Gabriel Duhre (about 1680-1739), who visited Uppsala in 1724-25 and gave a lecture course.
- Celsius had been so unhappy with the mathematics teaching that he thought he would specialise in law but this all changed after he attended Duhre's course.
- Celsius had, from the early 1720s, carried out observations for Erik Burman (1692-1729), Nils Celsius's successor as professor of astronomy in Uppsala, and, having been taught about meteorology and experimental physics by Burman, Celsius published his first two papers in 1724 both relating to barometers.
- After graduating, Celsius became a substitute lecturer in mathematics, while the professor, Samuel Klingenstierna (1698-1765), was abroad.
- Klingenstierna and Celsius had both been applicants for the vacant chair of mathematics in the University of Uppsala, but Klingenstierna had been appointed, mainly because of a very strong letter of recommendation from Christian Wolff, a pupil of Gottfried Leibniz.
- Klingenstierna had been awarded a travel grant to visit the European centres of learning and was not able to take up the chair of mathematics at Uppsala until he returned in 1731, so he had to pay Celsius out of his own pocket for taking on the duties of the chair of mathematics.
- When Erik Burman died in 1729, Celsius took over his lectures in astronomy in addition to substituting for the professor of mathematics.
- The University of Uppsala was now a very strong centre for mathematics and astronomy with Klingenstierna and Celsius both world-class scholars.
- One of the tasks Celsius and Meldercreutz undertook was to visit learned societies and academies, aiming to set up links with the Royal Society of Sciences in Uppsala.
- During his trips, Celsius made several astronomical observations with a quadrant he bought in Berlin.
- After initial complaints that the news was too old (due to the slow posting of the time), Celsius started admitting observations from the readers, but the magazine stopped publication two years later after probably 45 issues.
- When Celsius arrived at Paris in late 1734, the scientific community was in the middle of a discussion about the shape of the Earth.
- The Academy asked Celsius to make the corresponding observations in Sweden but he had to turn down their request since he had neither the financial support nor the necessary instruments.
- Celsius, therefore, joined the Arctic Circle expedition, headed by Maupertuis, and it was probably on his suggestion that it was decided to go to Lapland.
- Celsius was sent on a short trip to London to commission a zenith sector from Graham, a revered instrument maker of the time.
- During his visit to London, Celsius attended a meeting of the Society of Antiquaries and published in Philosophical Transactions of the Royal Society.
- In addition to Celsius and Maupertuis, the mathematicians and astronomers Alexis Clairaut, Pierre Charles Le Monnier (1715-1799) and Charles-Étienne Camus were among the party.
- Celsius took part in the debate which followed, where Jacques Cassini and his followers put into question the accuracy of the observations made during the expedition.
- Celsius defended them in "De Observationibus Pro Figura Tellius Determinanda in Gallia Habitis, Disquisitio" Ⓣ(Disquisition on observations made in France for determining the shape of the earth) (1738), rebutting all their allegations.
- In it, Celsius shows that, in his previous measures of the meridian in France, Cassini had committed bigger mistakes than the ones he was accusing them of having made.
- Due to the success of the Lapland expedition, Celsius received a pension of 1000 livres per year from the French government.
- When he returned to Uppsala, Celsius worked to improve the standing of astronomy in Uppsala and Sweden, which had been in decline.
- When building a precise thermometer, Celsius invented a new scale divided into 100 degrees, with 0 being the boiling point of water and 100 its freezing point.
- To justify his choice, Celsius carried out a number of experiments: he showed the freezing point did not change when varying latitude or pressure and that the boiling point did not depend on the length of the boiling time or the origin of the water.
- Celsius had studied the numerous temperature scales of the time during his travels through Europe in hopes of creating an international scale.
- The scale, which he used for the first time on 25 December 1741, became widely utilised after his death, but its orientation was reversed (0 degrees Celsius is now the freezing point of water, while 100 degrees is its boiling point).
- Celsius continued his astronomical studies during his time in Uppsala, sometimes in collaboration with his contacts in Paris.
- Celsius served as secretary of the Royal Society of Sciences at Uppsala until his death and was responsible for reviving the Society when he returned to Uppsala in 1737 after the Lapland expedition.
- Celsius also endeavoured to create a star catalogue and for this he wrote "Constellatione Tauri" Ⓣ(The constellation of Taurus), 1743, and Constellatione Leonis Ⓣ(The constellation of Leo), 1741, among other works.
- For a long time Celsius hesitated unsure whether to set up the classical instrument for this work at the observatory or at his home in Sävja.

Born 11 November 1701, Uppsala, Sweden. Died 25 April 1744, Uppsala, Sweden.

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Astronomy, Origin Sweden

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive