Today in the history of astronomy, a proponent of relativity is born. A collaborator with Albert Einstein, Willem de Sitter published on relativity and the expanding universe. Credit: Public domain via Wikimedia Commons 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.…