Sitter, Willem de (vĭlˈəm də sĭtˈər) [key], 1872–1934, Dutch astronomer and mathematician. He was professor from 1908 at the Univ. of Leiden and in 1919 became director of its observatory. His early work on the motions of Jupiter and its satellites contributed to the downfall of the pre-Einstein celestial mechanics. Using Einstein's formulation of relativity, he theorized that space cannot be in a stable equilibrium, and he concluded that the universe is expanding. He suggested a dynamic universe in which there is motion but no matter, in contrast to Einstein's static universe containing matter but no motion. In the combined Einstein–de Sitter model, the universe is expanding at a decreasing rate that approaches zero. De Sitter's works in English include Kosmos (1932) and The Astronomical Aspect of the Theory of Relativity (1933).
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