Born Johannesburg, South Africa, 28 July 1905
Died Johannesburg, South Africa, 16 May 1979
An indefatigable visual observer of southern-hemisphere double stars, William Stephan Finsen was director of the Union Observatory (later the Republic Observatory) in Johannesburg, South Africa. Born in Parktown, Johannesburg, to Danish immigrant parents, Finsen was educated at King Edward VII High School, the University of South Africa, and the University of Witwatersrand. He began his career at the Union Observatory in 1924 as a volunteer assistant under Robert Innes and worked his way up the ranks eventually to become its director, with the title Union Astronomer, in 1957. The visual measurement of double stars with the observatory’s 26.5-in. Grubb refractor comprised much of his work; he designed and built an eyepiece interferometer, which allowed him to detect pairs too close otherwise to be resolved. Finsen examined over 8,100 stars in his quest for binaries. He is credited with the discovery of 73 double stars; he calculated orbits of 12 of them. Finsen was also engaged in planetary astronomy. During the 1939, 1954, and 1956 oppositions of Mars, he took roughly 24,400 color photographs of the planet. His images were some of the best available before the Mariner IV flyby in 1965.
When South Africa left the British Commonwealth and declared itself a republic in 1961, the Union Observatory was renamed the Republic Observatory. Finsen was the first and only Republic Astronomer. Upon his retirement in 1965, the South African government chose not to appoint a successor; in the following years the operations of the observatory were curtailed until it was finally closed in 1971. Along with his predecessor Willem van den Bos , Finsen waged a spirited but ultimately unsuccessful campaign for the continuance of the Republic Observatory’s double star program. Although by this time the visual observation of double stars was increasingly perceived as obsolete, Finsen was convinced of its value to astrophysics, particularly in refining the empirical mass-luminosity relation. To the white South African public Finsen was a leading ambassador for his science. In the 1950s and 1960s, his voice was frequently heard on SABC radio, and he was instrumental in coordinating the Moon watch program at Johannesburg. The Astronomical Society of Southern Africa awarded him its Gill Medal in 1967. Asteroid (1733) Finsen is named in his honor.