André-Louis Danjon

Life and work

Danjon was born in Caen to drapers Louis Dominique Danjon and Marie Justine Binet. He studied at the Lyce Malherbe and then at the Ecole Normale Superieure, working at the observatory of the Societe Astronomique de France during that time. After graduating in 1914 he was conscripted into the army for World War I, serving under Ernest Esclangon and losing an eye in combat in Champagne. He received war honours in 1915 and in 1919 was appointed aide-astronome to Strasbourg University, becoming an observer at the Strasbourg Meridian observatory and working on its improvement. He helped establish the Observatoire de Haute-Provence, which became operational in 1923. Danjon devised a method to measure earthshine on the Moon's darkside using a telescope with a prism that split the Moon's image into two identical side-by-side images; by adjusting a diaphragm to dim one image until the sunlit portion matched the earthlit portion on the unadjusted image, he obtained a measurement for earthshine brightness. He recorded measurements using this method, now called the Danjon scale, from 1925 until the 1950s. He extended similar methods to study the albedo of Venus and Mercury, which became the subject of his doctoral dissertation Recherches de photometrie astronomique (1928) at Paris University. In 1930 he succeeded Ernest Esclangon as director of the Strasbourg Observatory and became a professor at Strasbourg University. During the German invasion in 1939 the faculty moved to Clermont-Ferrand near Vichy. He was arrested in November 1943, escaped being sent to Auschwitz, and was released in January. After World War II, Esclangon retired from the Paris Observatory and Danjon replaced him, teaching at the Sorbonne. In the 1960s he persuaded the government to establish the European Southern Observatories at La Silla and at Paranal, and supported the establishment of radio astronomy at Nancay in 1956.