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.