The Einstein - de Sitter Universe
By P.C. van der Kruit

Willem de Sitter was born in Sneek on May 6, 1872. After grammar school in Arnhem, he went to Groningen to study mathematics. However he did some experimental work for his physics curricula at the astronomical laboratory of the famous astronomer Jacobus C. Kapteyn. The latter was involved in measuring photographic plates from the Cape Observatory in South Africa, in collaboration with Sir David Gill to chart the southern skies. During a visit to Groningen Gill invited de Sitter to Cape Town, de Sitter accepted the invitation and decided to become an astronomer. He left in August of 1897 and among others worked on measurements of the four Galilean satellites of the planet Jupiter.

Towards the end of 1899 de Sitter returned to Groningen and obtained his Ph.D. degree there in 1901. The title was Discussion of Heliometer Observations of Jupiter's Satellites. In South Africa he also had met his wife Elanora Suermondt. The thesis, and in particular later work resulted in better orbits for these satellites, where also the mutual gravitational influences on each other s orbits were corrected on. This entailed new mathematical tools and approaches.

In 1908 de Sitter was appointed in Leiden as professor in theoretic astronomy and in 1918 he was appointed as director. Together with Kapteyn he reorganised the Observatory; where previously the work concentrated on fundamental observations of positions and motions of stars on the sky, the new observatory also concentrated from then on on astrophysics and theory. In spite of administrative duties, de Sitter still found time for studies on various aspects of astronomy and astrophysics. In particular his work on the General Theory of Relativity that Einstein had formulated, is universally known. He showed that the field equations that Einstein had formulated then very recently before also allowed a solution for the structure and evolution of the universe where it was expanding, contracting or oscillating. The first is now known to be the case. The most simple solution of the field equations for the expanding universe is called the Einstein-de Sitter Universe.

In 1919 de Sitter was operated for gallstones. However, the overdose of ether he was given resulted in a continuing poor health. Among others he suffered from tuberculosis, which made it necessary to spend two years in Arosa in Switzerland. He still succeeded in doing research and he arranged an agreement with the Union Observatory in Johannesburg, to jointly carry out observational studies. Also he took measures for two expeditions to Kenya to improve absolute positional measurements of stars on the sky. He received various important honors, among which the Gold Medal of the Royal Astronomical Society (1931) and the Bruce Medal of the Astronomical Society of the Pacific (1931). He was president of the International Astronomical Union from 1925 to 1928. After a brief illness he died on November 19 in 1934.