Born Caen, Calvados, France, 6 April 1890
Died Paris, France, 21 April 1967
André Danjon led the recovery of French astronomy to excellence after many decades of neglect and the two world wars fought on French soil. Danjon is remembered for his development of instruments and for his fundamental studies of the Earth’s rotation. As a result of his efforts to stabilize and expand the International Astronomical Union [IAU] during the troubled period after World War II, Danjon exercised substantial influence on twentieth-century astronomy.
The son of Louis Dominique Danjon and Marie Justine Binet, both drapers, Danjon was one of three siblings. His father was at first an accountant, which may perhaps explain Danjon’s lifelong interest in precision and exactness; characteristics he first evinced while a student at the Lycée Malherbe in Caen. In 1910, Danjon was accepted for admission to several of the major French institutions of which he chose the École Normale Supérieure [ENS]. During his studies at the ENS, Danjon spent many hours at the eyepiece of the refractor at the amateur observatory of the Société Astronomique de France. He graduated as agrégé de sciences physiquesin 1914.
When World War I broke out immediately after his graduation from the ENS, Danjon was mobilized and assigned to the sound-ranging service then under the command of astronomer Ernest Esclangon . Danjon lost one eye in combat in the Champagne region but remained in active service, receiving the Croix de Guerre avec Palmesand Chevalier de la Légion d’honneurin 1915.
After the war, in 1919 Danjon accepted a government appointment as aide-astronometo a group of high-level teachers sent to the university in Strasbourg, located in the historically contested region of Alsace-Lorraine, which had been ceded to France by Germany as part of the Versailles Treaty. Danjon took up duties as an observer in the Strasbourg meridian service but soon realized the inadequacy of both the century-old equipment and the procedures. His efforts to upgrade the Strasbourg meridian service, and similar later efforts at Paris Observatory, stimulated Danjon’s creative instincts for instrumental development.
In 1923, Danjon assumed the additional responsibility of conceptualizing a new observatory for astrophysics, the Observatoire de Haute-Provence in southeast France, which opened in 1936.
In parallel with these activities, Danjon continued to pursue physical observations of celestial objects. Using his invention, the photomètre à œil dechat (the cat’s eye photometer), Danjon made studies of the earthshine reflected by the dark side of the Moon. His studies were extended to include the albedo of Venus and Mercury as a function of their phase. This work formed the basis for his doctoral dissertation, entitled Recherches de photométrie astronomique, accepted in 1928 at Paris University. Danjon was then appointed adjoint astronomer at Strasbourg.
In 1930, Danjon succeeded Esclangon as director of the Strasbourg Observatory when the latter became director of the Paris Observatory. Soon thereafter, Danjon became a full professor, and in 1935, he was appointed dean of the Strasbourg faculty of sciences.
In 1939, German aggression forced the relocation of the entire university faculty including Danjon to Clermont-Ferrand near Vichy, France. Acting as the university rector, Danjon opposed the military use of the university campus, which resulted in his being arrested and jailed in late November 1943. Many of the professors and students arrested in this sweep were sent to Auschwitz, though Danjon and other docents of the university escaped that fate. Released in the following January, Danjon did not recover his position at the university until November 1944. When Esclangon retired in 1945, Danjon replaced him as the director of the Paris Observatory.
On his arrival in Paris, Danjon was faced with the urgent need to restore French observatories. More importantly, Danjon’s task should be viewed as restoration of French astronomy, which like many other sciences had been deeply diminished by two successive wars on French territory. As a teacher in La Sorbonne, Danjon had a deep influence over his students due to his very clear presentations and a fondness for astronomy that he demonstrated in his courses. When Henri Mineur died in 1954, Danjon assumed the directorship of the Astrophysical Institute of Paris in addition to the Paris Observatory. Danjon occupied many other administrative positions, always showing a great realism as an administrator. Among them are president of the International Committee on Weights and Measures (1954) and founding president of the French Association for Numerical Computation (1957). He was also a member of the Bureau of Longitudes and was elected to the French Academy of Sciences in 1948.
Danjon’s instrumental research focused on astronomical applications of double-image or Wollaston prisms. By 1952, he developed the prototype of a prismatic astrolabe equipped with an impersonal micrometer (now commonly referred to as the astrolabe de Danjon). In parallel to photographic zenith tubes, a total of 45 of Danjon’s astrolabes were in service for time and latitude determinations at various locations until 1987. On a good night, the Danjon instrument was capable of determining time with an accuracy of 4 ms and latitude to 50 mas.
While director in Paris, and still observing in the mid 50s, Danjon established or improved several domains of French astronomical research, expanding coverage to a majority of fields in modern astronomical activities. In 1956, his efforts led to the establishment of the Radio Astronomy Station at Nançay, situated far away from industrial and human-made noise, in Sologne, the Cher department. When his European colleagues suggested establishing a new European observatory in the Southern Hemisphere, as Adrian Blaauw later noted, it was Danjon who persuaded the French government to take part in the project, leading to the development of European Southern Observatory stations at La Silla and at Paranal.
Danjon retired from his position in 1963 and died in April 1967 just before the tercentenary of the Observatoire de Paristhe following June.
For a physicist in the service of astronomy with many administrative duties, the volume of Danjon’s publication deserves mention. He published many fundamental papers, for example, on the influence of the Earth’s atmosphere on the variations of its rotation; his early works on the reflecting power of the Earth were reconsidered favorably in 1980, remarkable in view of the half a century that had passed. The Danjon scale is used to this day to rate the brightness of lunar eclipses.
A talented popularizer, Danjon’s public lectures and his papers in l’Astronomie, the magazine of the Société Astronomique de France, now constitute useful sources for those who want to study the evolution of astronomical research during the decades for which he had major responsibilities. An amateur astronomer in his youth, Danjon remained very active within the Société Astronomique de France, encouraging cooperation between its amateur members and professional astronomers. He considered popularization of astronomy a duty for researchers.
Danjon was a corresponding member of astronomical societies in Belgium, Portugal, the United States, Italy, and Great Britain and served as the IAU president from 1956 to 1958. The Royal Astronomical Society [RAS] of London awarded Danjon its highest honor, the RAS Gold Medal, in 1958; that same year he served as the RAS Darwin Lecturer. In 1954, Danjon was Commandeur de la Légion d’honneurafter being Officierin 1946. During the second half of the twentieth century, progress in French astronomical science owed a great deal to Danjon.
In 1919, Danjon married Madeleine Renoult, and they had four children; she died in 1965.