Sitter, Willem de

Sitter, Willem de (18721934), a Dutch astronomer. He was a pioneer in applying Albert Einstein's theory of relativity to astronomy and developed a model of an expanding, curved universe. De Sitter's theoretical model led to further development of the theory of the expanding universe. De Sitter studied at the University of Groningen and became a professor at the University of Leiden in 1908, Later, he served as director of its observatory.