Sitter, Willem de
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Related to Sitter, Willem de: Karl Schwarzschild, De Sitter space, Lemaitre
Sitter, Willem de(vĭl`əm də sĭt`ər), 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).
Sitter, Willem de
Born May 6, 1872, in Sneek; died Nov. 19, 1934, in Leiden. Dutch astronomer.
De Sitter graduated from the University of Groningen. In 1897 he began work in photometry at the observatory on the Cape of Good Hope, where he developed a theory for the motion of the first four satellites of Jupiter. In 1908 he became a professor of astronomy and in 1919, director of the observatory of the University of Leiden. De Sitter created his own fundamental system of astronomical and geodetic constants. His works on the theory of relativity served as an impetus for the organization of an expedition to observe the solar eclipse of 1919. It was during this eclipse that the deviation of light rays passing near the sun was detected, a phenomenon that had been predicted by Einstein.