Anders Celsius

Anders Celsius was born at the 27th of January 1701 in Uppsala.
After his education time in this town north of Stockholm he became professor for astronomy already in 1730. At this time there was no larger observatory anywhere in Sweden. Therefor Celsius began a round trip to some of the famous European astronomy sites in 1732. He came from Nuremberg and Rome to Paris in 1734.

Director of the observatory there, founded in 1672, was Jaques Cassini (1677-1756) son of Jean-Dominique Cassini (1625-1712). At this time a disput between English and French astronomer about the shape of the Earth was actual. To find the true answer to this question expeditions had to be sent to the "ends" of the world to measure exactly the local positions. The expedition to the north was commissioned to Pierre Louis de Maupertuis (1696-1759), which Celsius joined as an assistant. The journey to Lapland, the very northern part of Sweden lasted from 1736 to 1737 and with the other measurements it confirmed the theory of Newton about the flattening of the Earth poles.

After the expedition Celsius returned to Uppsala and worked on the erection of an observatory, which was finished in 1740. At and after the travel to Lapland and back Celsius was one of the first to examine the changes of the Earth magnetic field at the time of a Northern light. He was also one of the first measuring the brightness of stars with measurement tools. After the completion of the first Swedish observatory he became the director of it. Celsius became famous for his recommendation in 1742 to divide the temperature scale of a mercury thermometer at 760mm mercury air pressure into 100 degrees, where 100 is the frozing point and 0 the boiling point of water. Because of the detailed fixation of the measuring circumstances and methods this definition was more exact and enabled a better temperature measurement as with the actual practices, introduced by the glass manufacturer Gabriel Daniel Fahrenheit (1686-1736) and the aristocrate and biologist René-Antoine Ferchault de Réaumur (1683-1757). Later the reversion of the Celsius scale with 0 as the frozing point and 100 as the boiling point of water was introduced, and with this modification it became widely spread.

Celsius was an active supporter for the introduction of the Gregorian calendar reform in Sweden. This was already tried in 1700, but it was planned to introduce the modifications of the date stepwise by dropping the leap days from 1700 to 1740. When 1704 and 1708 were declared as leap years by error, in 1712 Sweden returned to the Julian calendar. The engagement of Celsius was only fruitful after his death. Celsius died early in 1744 in his hometown. The Gregorian calender was introduced in Sweden in 1753, doing it by dropping the supernumerary 11 days at once. (Source: calendar.faq)

Celsius thermometer (attached to a barometer) made by J.G. Hasselström, Stockholm, late 18th century.

Anders Celsius early became engaged in the general problem of weights and measures, including temperature measurements. Already as a student he assisted the astronomy professor Erik Burman in meteorology observations. At that time there existed a large variety of thermometers with different scales. Perhaps he already at this stage realized the necessity of a common international scale.

A temperature scale must be based on one or two standard temperatures, called fixed points. For those it was natural to choose temperatures within the temperature domain of practical interest, i.e. from about plus forty to minus twenty in modern Celsius degrees. Thermometers were simply used in meteorology, in horticulture, and sometimes for indoor use. As fixed points one could use the human body temperature or temperatures of local origin such as the observatory cellar in Paris or the highest temperature in sunshine in London. Of course also the freezing and boiling points of water were used, but it was not self-evident that they really were universal and e.g. independent of the geographic latitude.

The position of the degree zero on a temperature scale has created a lot of discussions. The French Réaumur scale had zero at the freezing point but as regards other scales one placed zero outside the ordinary temperature region, thus avoiding the mixture of positive and negative numbers. The zero could be placed at a low temperature, a method used by the Danish astronomer Ole Rømer and then adopted by Fahrenheit. Celsius was accustomed to the Réaumur thermometer, but he also used a thermometer made by the French astronomer Joseph-Nicolas Delisle with zero at the boiling point, thus creating a reversed scale with increasing numbers for decreasing temperatures, but avoiding negative numbers.

From the scientific point of view the most important contribution to the modern temperature scale is due to Celsius because of his careful experiments on the fixed points. The direction of the scale comes in second place, and was hardly a great intellectual achievement.


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