André Danjon (1890–1967) was one of France's most eminent twentieth-century astronomers. Born in Caen, he successively directed the Strasbourg Observatory (1929–1945) and the Paris Observatory (1945–1963), which he profoundly modernised. Elected to the Académie des Sciences in 1948, he presided over the International Astronomical Union (1955–1958). He invented the impersonal astrolabe (the "Danjon astrolabe"), a high-precision instrument for measuring time and Earth's rotation, and devised the Danjon scale for the photometric measurement of lunar luminosity during eclipses, still in use today. Actively involved in artificial satellite observation during the International Geophysical Year (1957–1958), he oversaw the revision of the Astronomie Populaire Camille Flammarion series and contributed several pedagogical syntheses on nascent astronautics.