Dutch astronomer, mathematician, and physicist, who contributed
to the birth of modern cosmology. He was influential in English-speaking
countries in bringing the relevance of the general theory of relativity
to the attention of astronomers.
De Sitter was born in Sneek, Friesland, and studied at the University
of Groningen and the Royal Observatory, Cape Town, South Africa. He
was professor of theoretical astronomy at the University of Leiden from
1908 as well as director of its observatory from 1919.
In 1911 de Sitter outlined how the motion of the constituent bodies
of our Solar System might be expected to deviate from predictions based
on Newtonian dynamics if Albert Einstein's special relativity theory
were valid. After the publication of Einstein's general theory of relativity
1915, de Sitter expanded his ideas and introduced the 'de Sitter universe'
(as distinct from the 'Einstein universe'). His model later formed an
element in the theoretical basis for the steady-state hypothesis regarding
the creation of the universe. He presented further models of a nonstatic
universe: he described both an expanding universe and an oscillating