MINNAERT, MARCEL GILLES JOZEF

(b. Bruges, Belgium, 12 February 1893; d. Utrecht, Netherlands, 26 October 1970)

astronomy.

Minnaert one of the pioneers of solar research in the first half of the twentieth century, was professor at the University of Utrecht and director of its observatory from 1937 to 1963. His parents were teachers at normal schools and many of his other relatives were involved in teaching, which background undoubtedly determined his later interest in science and education. He studied biology at the University of Ghent and in 1914 defended—with the highest distinction—his doctoral thesis, “Contributions à la photobiologie quantitative.”

At that time the University of Ghent, although situated in the heart of the Dutch-speaking part of Belgium, used French as the language of instruction, as did the other universities and most of the secondary schools in Flemish Belgium. Minnaert, who gradually realized that the linguistic problem was also a social problem related to the underdeveloped status of Flanders, joined associations of Flemish students and intellectuals who sought political equality and, later, relative independence (federalism) for both parts of Belgium. They also wished to convert the University of Ghent to the Dutch language. During the German occupation of Belgium in World War I, the latter goal was attained.

The urgent need for teachers at the new Flemish university induced Minnaert to go to Leiden in 1915–1916 to study physics. After his return to Ghent he was named associate professor of physics and remained in that post until 1918. At the end of the war those who had cooperated in the linguistic reform of the University of Ghent were accused of collab oration with the Germans, and many received long prison sentences. In order to escape that fate Minnaert moved to Utrecht, to which place he was attracted by the technique of objective photometry, then being developed at its physics laboratory by W. H. Julius, Ornsiein, and Moll. He readily understood the importance of the technique because of his previous experience in phutobiology, in which specialty the lack of quantitative measures was deeply felt. The director, W. H. Julius, had just set up a solar spectrograph—at that time the third in the world—intending to apply spectrophotometric techniques to the solar spectrum. Minnaert became interested in this work, and after Julius’ death in 1924 he assumed the main responsibility for solar research at Utrecht. In 1925 he defended—cum laude—another thesis, this time in physics:“Onregelmatige straalkromming”(“Irregular Refraction of Light”).

At that time the basic requirements were available for quantitative research in solar physics: Bohr’s atomic model, Saha’s ionization law, the developments of quantum theory, and the new technique of quantitative spectrophotometry made possible the quantitative interpretation of the solar spectrum. Minnaert developed the concepts “equivalent width” and “curve of growth”; the theory of weak lines was carried further; and the intensity measurements of sunspots made possible the physical interpretation of these phenomena. This work, performed in the physics laboratory at Utrecht, culminated in 1940 in Photometric Atlas of the Solar Spectrum (in collaboration with Houtgast and Mulders), which is still a standard reference.

In 1937 Minnaert was named director of the University of Utrecht observatory, which he transformed into an astrophysical institute’ devoted mainly to the investigation of solar and stellar spectra. Yet his interests were wider than the sun: he studied comets and gaseous nebulae and was involved in lunar photometry; and during the last few years of his life he was a member of a working group of the International Astronomical Union concerned with naming the newly discovered formations on the hemisphere of the moon that is not visible from the earth. In 1970, two months before his death, Minnaert was elected president of the Commission for the Moon of the International Astronomical Union. His broad interest in science and nature led him to write these books: De Natuurkunde ran het vrije veld (“Physics of the Open Field”), translated into many languages; Dichters over Sterren (“Poets on Stars”); and De sterrekunde en de mensheid (“Astronomy and Mankind”).

In 1928 Minnaert married Maria Boergonje Coelingh, who defended her thesis in physics in 1938; they had two sons. Philosophically Minnaert defended determinisim; politically he had strong left-wing sympathies but was too committed to science to link himself to any political party. Yet his political ideas were sufficiently known to the Germans for him to be imprisoned in 1942–1944.

Minnaert spoke ten languages fluently and could read in even more. Even so he was an enthusiastic defender of Esperanto—attracted by the simplicity and regularity of this artificial language which, he felt, could be of great importance for both scientific and social communication. He loved music and painting and cultivated both actively. Above all, Minnaert had a strong interest in humanity and its problems. He was admired and loved by his friends, students and co-workers, and respected by those who did not agree with his social or political ideas, In the last few years of his life he was very active in an international group purchasing books for the University of Hanoi, North Vietnam.

Minnaert was a member of the Royal Netherlands Academy of Arts and Sciences, of the Royal Belgian Academy of Science, Letters and Fine Arts, of the Kungl, Vetenskapsamhället of Uppsala, and of the Instituto de Coimbra; and associate of the Royal Astronomical Society of London. In 1947 he received the gold medal of the Royal Astronomical Society and in 1951 that of the Astronomical Society of the Pacific (Bruce Medal). He held honorary doctorates from the universities of Heidelberg, Moscow, and Nice.

BIBLIOGRAPHY

Minnaert’s main publications were his diss., Onregelmatige straalkromming (Utrecht, 1925); De Natuurkunde van de zon (The Hague, 1936); De betekenis der zonnephysica voor de astrophysica (Utrecht, 1937); De Natuurkunde van het vrije veld (Zutphen, 1937); Photometric Atlas of the Solar Spectrum (Amsterdam, 1940), with G. F. Mulders and J. Houtgast; De sterrekunde en de mensheid (The Hague, 1946); Dichters over Sterren (Arnhem, 1949); and Practical Works in Elementary Astronomy (Dordrecht, 1969). He was the author of many papers, of which a review is given in M. Minnaert, “Forty Years of Solar Spectroscopy,” in C. de Jager, ed., The Solar Spectrum, the proceedings of a symposium held at the occasion of the sevetieth anniversary of the birth of M. G. J. Minnaert (Dordrecht, 1965).

Minnaert also prepared the articles on Hoek, Kaiser, Hortensius, W. Julius, Pannekoek, and Stevin for this Dictionary.

C. de Jager