Uppsala University: Office for History of Science

nr 26 - 1997

Astronomy in Sweden 1860-1940

by Gustav Holmberg

The intellectual geography of Swedish astronomy and its international contacts has changed since the 19th century. Then, many Swedish astronomers travelled to Germany, with its excellent observatories and well-known astronomers. The observatories at Uppsala, Lund and Stockholm had many instruments from the leading German instrument makers. Another route was eastward to the Russian central observatory in Pulkovo, near S:t Petersburg. Oscar Backlund was director of the Pulkovo observatory from 1895 until his death in 1916. He had arrived in Pulkovo in 1878, shortly after Bernhard Hasselberg, who was in Pulkovo between 1872 and 1889, when he got a post as physicist at the Royal Academy of Sciences in Stockholm. Hasselberg's stay in Pulkovo was important because he picked up skills in astronomical photography and spectroscopy that simply did not exist in Sweden at the time: the Russian obervatory was very well furnished with resources for the latest astronomical technologies. Several other Swedish astronomers also worked in Pulkovo for longer or shorter periods of time. More and more during the 20th century, astronomers turned westward. Large US observatories became important astronomical centres, and early in the century, English became the preferred language for PhD dissertations in astronomy.

Spectroscopy was an important technology in the birth of astrophysics. A physicist at Uppsala University, Anders Ångström, made a survey of the lines in the solar spectrum. At the observatory in Lund, the astronomer Nils Dunér started working with spectroscopy in the late 1870's. Dunér had earlier worked with classical astronomy: celestial mechanics and double stars. In 1878 he bought his first spectroscope from the German instrument maker Heustreu, the following year a spectroscope made by Merz of Munich. Dunér began a survey of the spectra of red stars, these being singled out partly because of the idea that the red stars were at a more advanced evolutionary level. Dunér made a catalogue of the spectra of red stars, published in 1885.

After his study of red stars, Dunér started work on the solar spectrum. He aimed at measuring the rotation rate of the sun by measuring the doppler shifts of the solar lines. Dunér had an instrument constructed around gratings made by the American physicist Henry Rowland. The mechanical parts of this very large spectroscope were made by Jürgensen in Copenhagen, a firm that made instruments for scientific, military and industrial use. Dunér measured the displacement of iron lines in the solar spectrum in relation to stationary lines produced by oxygen in the Earth's atmosphere. The careful measurements made with the large instrument produced a view of how the sun rotates in different latituds with an accuracy not obtained by earlier observers. In Dunér's and Hasselberg's correspondence, it it evident that Hasselberg's spectroscopic experience at Pulkovo was important for Dunér.

Photography was introduced in Swedish astronomy in the 1880's. After the introduction of the dry plate, astronomers in larger numbers began using the technology. Early users of photography in Sweden included Hugo Gyldén, a well-known specialist in celestial mechanics working at Stockholm Observatory. Here Gyldén began using photography during the 1880's, photographing stars down to a limiting magnitude of 13 in initial tests with the 3-inch Steinheil astrograph. When Dunér became professor at Uppsala University in 1889, he introduced the photographic technology, and aimed at having a modern observatory built. He managed to get funds for a large double refractor with one visual and one photographic tube, as well as visual and photographic spectroscopic equipment. Later on, Dunér's successor Östen Bergstrand worked in photographic studies of stellar colours. Photographic astronomy was ideally suited for working with solar eclipses. For instance the total solar eclipse of August 1914 was photographically studied by several Swedish astronomers.

Photography entailed a division of labor in astronomy. The photographic astronomers exposed plates that sometimes registered thousands of celestial objects that later could be analysed, sometimes in other institutional contexts. The amount of data available to astronomers grew, and data became more mobile. One way to use this data was stellar statistics, a type of astronomy that used large datasets and analysed them statistically to get a picture of how stars were distributed throughout space. Stellar statistics was represented in Sweden most prominently at Lund Observatory, where the so-called Lund school of stellar statistics was formed around C.V.L. Charlier from about 1910. Charlier and his disciples made models of the distribution of stars in space based on emprirical materials, often observed in other parts of the world. The masses of data were handled by several women, working as computing assistants.

Charlier and his colleagues were also active in utilising the statistical tools in non-astronomical ways. They collaborated in official reports on several subjects requiring their statistical skills. Two of the leading statisticians in the 1930's, Josua Linders and Sven Wicksell, both professors of statistics, had studied under Charlier. The statistical methods were used in the modernisation of governement and industry, and also came into use in various sciences. Linders worked for a while as statistician in the Eugenics Institute in Uppsala, and another Charlier student, Olof Åkesson, became a prominent figure in the area of public health insurance and retirement schemes.

Charlier also studied large-scale cosmological models. He argued that the universe was infinite. In two papers published in 1908 and 1922 he showed that if matter was distributed in the universe in a certain way, the problem of Olber's paradox was solved. The paradox states that if the universe is infinite and matter on the whole is distributed evenly throughout space, then the line of sight would in every direction reach a star, and hence the sky would be lit up like the sun in every direction. Charlier argued that the paradox was resolved by postulating a certain distribution of matter he called the convergence criterion of the universe.

Svante Arrhenius was also interested in the large-scale structure of the universe, and, like Charlier, favoured infinite models of the cosmos. Arrhenius, a chemist and physicist who won the Nobel chemistry prize in 1903 for his theory of electrolytic dissociation, argued that the universe was eternal. He thought the the heat death was kept in check by mechanisms in the universe in which the energy radiated from the stars was somehow stored in gaseous nebulae, later to be released again when the nebulae formed new stars. Arrhenius also wrote on the origin of life on Earth, favouring the panspermia theory, the idea that life came to the Earth from the universe. Arrhenius renewed the theory - a theory which had had several earlier proponents - in that he proposed a new transportation method. Earlier thinkers had hypothesised that "the seeds of life" came to the Earth riding on meteorites, whereas Arrhenius invoked radiation pressure. He calculated that "seeds of life" were just about the right size to be able to ride on the radiation pressure from the stars. Before utilizing radiation pressure in this way, Arrhenius had used the effect for explaining several other astrophysical and geophysical effects, like the aurora borealis and the structure of the comets. Arrhenius' theory, which used radiation pressure as an important cosmological principle, had been developed in a series of papers from the mid-1890's in collaboration with the meteorologist Nils Ekholm, using the large datasets collected by several scientific expeditions to the arctic. The Humboldtian discipline of cosmic physics constitutes a crucial background to Arrhenius' work in cosmology and astrophysics. In 1903 Arrhenius published a Lehrbuch der kosmischen Physik. He was, however, most well known for the popularizations of his cosmological views, the prime example being Världarnas utveckling published in several editions from the first in 1906 to the last in 1929, edited after Arrhenius' death by Knut Lundmark.

Knut Lundmark was a somewhat less speculative scientist than Arrhenius. In an early work published in 1919 he argued that the spiral nebulae were distant stellar systems. He expanded his arguments in his PhD dissertation the following year. In those days, before Edwin Hubble's discovery of cepheids in the Andromeda Nebula, no consensus as to the nature of the spiral nebulae had been reached. Lundmark worked at the Mount Wilson and Lick observatories for two years in the early twenties, and in 1929 he became professor of astronomy in Lund, winning in the extremely close and heavily-contended competition with the other candidates for the position, Walter Gyllenberg and Gunnar Malmquist, who both belonged to the Lund school of stellar statistics. In the fight for the Lund professorship, two different styles of scientific reasoning met. The Lund school accused Lundmark's work of a lack of precision, since he didn't employ mathematics extensively. Lundmark accused the stellar statisticians of being more interested in the mathematical details than in astronomical facts.

The Lund Observatory had small and old instruments, unsuited for the kind of astronomy Lundmark was interested in. Lundmark and his pupils worked instead on observations performed in other places. Photographic plates exposed at observatories in the US, Germany, Egypt and so on were analysed in Lund. For a long time, Lundmark worked on the problem of nebulae. He intended to produce the Lund General Catalogue of Nebulae, in which he was to collect the known facts about a large number of nebulae, about 40,000 objects. The project was never finished. The task was too big and Lundmark was unable to raise the funds necessary for such a big undertaking. Lundmark also wrote much popular astronomy and astronomy history, in the popular press and in books. He published on a diverse number of subjects: the history of Greek astronomy, the astronomy of the Bible, Tycho Brahe, botany, August Strindberg etc.

During the 1920's, several astronomers and astronomy enthusiasts argued for a modernisation of Swedish astronomy. Plans were made for a large modern observatory in Sweden, an observatory in the southern hemisphere, more computers (which then meant humans performing routine calculations) and so on. Some of these plans were realised. Bertil Lindblad became leader of the Stockholm Observatory after Karl Bohlin retired in 1927, and some years later, in 1931, the new Stockholm Observatory was inaugurated in Saltsjöbaden, outside the city. Here a number of modern instruments were installed, amongst them a 1-meter reflector and a large astrograph. Lindblad was mostly interested in theoretical astronomy, working on the way stars move in the galaxies, but the observatory he headed was far better equipped than any of the other Swedish observatories.

The changing status of Swedish astronomy evident in the construction of the Saltsjöbaden Observatory is also evident in the fact that the 1938 General Assembly of the International Astronomical Union was held in Stockholm, indicating the respect that Swedish astronomy was accorded in the international astronomical world. However, a wider internationalisation, with cooperation at observatories in the southern hemisphere, had to wait until after the war.