Willem de Sitter (Sitter Willem de)

(1872-1934)
Born May 6, 1872 in Snacks. He graduated from Groningen University, worked at the astronomical laboratory. After two years of work mathematician calculating Observatory Cape of Good Hope (1897-1899) became an assistant of the Astronomical Laboratory at Groningen, and in 1908 - professor of astronomy at Leiden University. Since 1919 Sitter - Director of the Leiden Observatory.
Work Sitter devoted positional astronomy, photometry of stars and cosmology. After analyzing data on the motion of the satellites of Jupiter, obtained by himself and other astronomers at the Cape of Good Hope, he created a new theory of motion, which takes into account perturbations of different nature. This theory is used at present to calculate the motion of Jupiter's moons. Sitter belong on the harmonization of the various astronomical and geodetic constants. He studied the unevenness of the earth's motion and explained the slowing of its rotation by tidal friction.

Work Sitter on the theory of relativity, presented by Royal Society of London in 1916-1917 attracted the attention of the scientific world to the general theory of relativity Einstein. Sitter created one of the first relativistic cosmological theories, which served as a starting point for further theory of nonstationary Universe (Fridman, 1922; J. Lemaitre, 1927). In 1931 Sitter arrived in the U.S., the winter spent in the Mount Wilson observatory, watching along with Einstein on the movement of distant galaxies, and discussing the theory of the universe's expansion. Lectures given Sitter in the U.S. University of California, were published with additions in 1933. Died Sitter in Leiden, 20 November 1934.