Anders Celsius

Anders Celsius was born in Uppsala, Sweden, on November 27, 1701, into a household where science was the family trade. His father, Nils Celsius, held the chair of astronomy at Uppsala University, and both of his grandfathers had been professors there as well: Magnus Celsius in mathematics and Anders Spole in astronomy ^[1]. The family name itself came from an earlier generation of clergymen who Latinized the name of their home parish, Ovanaker in the province of Halsingland, drawing on the Latin word for a mound or hill ^[2].

Growing up amid lecture notes, instruments, and university politics, the young Celsius showed an early gift for mathematics. He entered Uppsala University, where he studied astronomy, mathematics, and experimental physics, and he advanced quickly through the academic ranks of a small institution where his relatives had taught for decades ^[1]. Sweden in these years was recovering from the long wars of Charles XII, and its universities were only beginning to absorb the observational science flourishing in Paris, London, and the German states.

By 1725 Celsius had become secretary of the Royal Society of Sciences in Uppsala, the oldest scientific society in Sweden, a post he retained for the rest of his life ^[3]. The position placed him at the center of Swedish scientific correspondence while he was still in his early twenties, and it gave him the administrative experience he would later use to argue for a national observatory.

In 1730, at the age of 28, Celsius was appointed professor of astronomy at Uppsala University, the same chair his father and grandfather had occupied ^[1]. He understood, however, that Swedish astronomy lagged behind the observatories of continental Europe, and between 1732 and 1735 he undertook a long tour abroad. He visited observatories and instrument makers in Germany, Italy, and France, meeting many of the leading astronomers of the day and studying how the best-equipped institutions were run ^[3].

During these travels he published, in Nuremberg in 1733, a collection of 316 observations of the aurora borealis, gathered by himself and by other observers over the previous decades ^[2]. The northern lights were a natural subject for a Swedish astronomer, and Celsius treated them not as an omen but as a phenomenon to be catalogued and measured. Working later with his assistant Olof Hiorter, he helped establish that displays of the aurora were accompanied by disturbances in the Earth's magnetic field, observed as deflections of a compass needle, an early step toward the science of geomagnetism ^[3].

The journey abroad also brought him into contact with the great scientific controversy of the era: the shape of the Earth. Followers of Isaac Newton argued that the planet must be flattened at the poles, while the French astronomer Jacques Cassini and his supporters held that it was elongated instead. Settling the question required measuring the length of a degree of latitude at very different distances from the equator, and the French Academy of Sciences organized expeditions to do exactly that ^[4].

Celsius joined the French expedition led by the mathematician Pierre Louis Moreau de Maupertuis, which traveled in 1736 to the Torne valley in the far north of Sweden, a region then commonly called Lapland ^[4]. His local knowledge, his standing as a professional astronomer, and his connections in Sweden made him a valuable member of the party. The team spent months in harsh conditions, hauling instruments up forested hills, triangulating across the frozen Torne river, and measuring a baseline on the ice during the Arctic winter.

The expedition's measurements, compared with a degree of latitude measured in Peru by a companion expedition, showed that a degree near the pole was longer than one near the equator. This confirmed Newton's prediction that the Earth is an oblate spheroid, slightly flattened at the poles ^[4]. Maupertuis became famous as the man who flattened the Earth, and Celsius shared in the credit; he later defended the expedition's results in print against critics from the Cassini camp ^[3].

The episode transformed his career at home. His participation in a celebrated international project gave weight to his long-standing request for proper facilities, and the Swedish government granted funds for an observatory in Uppsala. The Uppsala Astronomical Observatory, the first modern installation of its kind in Sweden, was completed in 1741 with Celsius as its director, equipped with instruments he had purchased during his years abroad ^[1].

Celsius is remembered above all for the temperature scale he proposed in 1742 in a paper read to the Swedish Royal Academy of Sciences, titled in translation "Observations of two persistent degrees on a thermometer" ^[5]. Thermometers of the period were notoriously inconsistent: makers used different fluids, different fixed points, and different divisions, so that readings from one instrument could rarely be compared with another. Celsius attacked the problem experimentally. He showed that the freezing point of water was effectively independent of latitude and atmospheric pressure, and that the boiling point, while it varied with pressure, could be corrected using the barometer ^[5].

On this basis he defined a scale with 100 degrees between the two fixed points. In his original version, 0 marked the boiling point of water and 100 the freezing point, so the numbers ran opposite to modern usage ^[5]. The scale was inverted to its familiar form shortly after his death; the botanist Carl Linnaeus used a reversed thermometer in 1745, and the instrument maker Daniel Ekstrom and others in Sweden adopted the same arrangement, though the exact credit for the inversion has been debated ^[6]. For two centuries the scale was widely known as the centigrade scale, and in 1948 an international conference on weights and measures formally renamed the unit the degree Celsius in his honor ^[6].

His achievements extended well beyond thermometry. Celsius was among the first astronomers to attempt to measure the brightness of stars with a tool rather than the naked eye, using a series of identical glass plates and counting how many were needed to extinguish a star's light, an early form of photometry ^[3]. He also made careful observations of eclipses and planetary positions, and he initiated one of the longest continuous series of meteorological and geophysical measurements in Sweden. With Hiorter he documented the connection between auroras and magnetic disturbances through simultaneous observations, including coordinated readings with observers in London ^[3].

One of Celsius's most original investigations concerned a puzzle familiar to Baltic fishermen and harbor towns: the sea seemed to be retreating. Rocks once used as seal-hunting perches stood above the waterline, and old harbors had become too shallow for boats. Celsius gathered testimony from coastal communities and marked reference lines on shoreline rocks so that future observers could measure the change precisely; one such mark, cut at Loevgrund near Gavle in 1731, still exists ^[7].

From this evidence he estimated that the water level was falling relative to the land by roughly a centimeter each year, and he interpreted the change as a diminution of the water itself ^[7]. The true explanation, that the Scandinavian landmass is slowly rising after being pressed down by the ice sheets of the last glaciation, a process now called post-glacial rebound, lay far beyond the geology of his time. Yet his method was sound: define a fixed reference, measure repeatedly, and let the record accumulate. The marks he and his successors made remain useful to geophysicists today ^[7].

This blend of patient measurement and openness to unexpected conclusions ran through everything he did. Celsius published works of popular science as well, including a book of arithmetic for Swedish youth, and he took part in the calendar debates of his day, supporting Sweden's eventual adoption of the Gregorian calendar ^[2].

The final years of Celsius's life were spent consolidating the institution he had built. From the new Uppsala observatory he maintained regular series of astronomical, magnetic, and weather observations, trained students, and continued his duties as secretary of the Royal Society of Sciences ^[1]. He never married, and contemporaries described a man absorbed in his instruments and his correspondence ^[2].

His health, however, was failing. Celsius contracted tuberculosis, the disease that killed so many scholars of the eighteenth century, and he died in Uppsala on April 25, 1744, at the age of 42 ^[1]. He was buried at Gamla Uppsala church beside his grandfather Magnus Celsius, a short distance north of the university town where he had spent nearly his whole life ^[2].

He did not live to see his temperature scale reversed into its modern form, nor the international adoption that would make his name a household word. At his death he was known chiefly as Sweden's leading astronomer, the builder of its first real observatory, and a respected participant in the Lapland expedition.

Few scientists have a legacy so visible in ordinary life. The Celsius scale is the standard temperature scale in nearly every country, used in weather reports, medicine, cooking, and science, and the degree Celsius is one of the units accepted for use with the International System of Units ^[6]. Anyone asking who was Anders Celsius usually meets the thermometer first, but the scale reflects a deeper contribution: the insistence that instruments must be calibrated against reproducible natural fixed points so that measurements made anywhere can be compared ^[5].

Among Anders Celsius achievements, historians of science also count the founding of the Uppsala Astronomical Observatory, the confirmation of the flattened Earth through the Lapland measurements, the joint discovery with Hiorter of the magnetic nature of auroras, and the shoreline marks that later helped reveal the slow rise of Scandinavia ^[3]. A crater on the Moon carries his name, as does the Celsius building in Uppsala where his observatory once operated ^[2].

Any Anders Celsius biography ultimately describes a bridge figure: a man who carried the observational standards of Paris and London into a small northern university and left it with instruments, data series, and habits of precision that outlasted him. Basic Anders Celsius facts, his birth in Uppsala in 1701, his professorship at 28, his hundred-degree scale of 1742, his death in 1744, sketch a short life. The measure of it is that people state temperatures in his name every day without thinking of the careful experimenter behind the word ^[6].