Dutch astronomer Willem de Sitter, born May 6, 1872, studied mathematics and physics at the University of Groningen, developing an interest in astronomy there and furthering it with work at Cape Observatory in South Africa. In 1908, he was appointed professor of astronomy at Leiden University, becoming the director of the Leiden Observatory in 1919.
Among de Sitter’s significant achievements in astronomy was his introduction of the general theory of relativity to British astronomers, bridging a breakdown in communication due to World War I. His work also inspired the famous 1919 eclipse expedition to prove relativity. In 1917, he proposed a model of the universe that extrapolated that relativity implied an expanding cosmos; in 1932, he would co-author a paper with Albert Einstein about the expanding universe. He also dedicated decades to refining classic measurements of the orbits and masses of Jupiter’s moons. He died in 1934 of pneumonia, at the age of 62.
