• Born: 6 May 1872, Sneek
• Died: 20 November 1934 (aged 62)
• Nationality: Dutch
• Spouce:
• Children: Aernout de Sitter

Born in Sneek, De Sitter studied mathematics at the University of Groningen and then joined the Groningen astronomical laboratory. He worked at the Cape Observatory in South Africa (1897–1899). Then, in 1908, de Sitter was appointed to the chair of astronomy at Leiden University. He was director of the Leiden Observatory from 1919 until his death.

De Sitter made major contributions to the field of physical cosmology. He co-authored a paper with Albert Einstein in 1932 in which they argued that there might be large amounts of matter which do not emit light, now commonly referred to as dark matter. He also came up with the concept of the de Sitter space and de Sitter universe, a solution for Einstein's general relativity in which there is no matter and a positive cosmological constant. This results in an exponentially expanding, empty universe. De Sitter was also famous for his research on the planet Jupiter.

Willem de Sitter died after a brief illness in November 1934

In honour of the 300th anniversary of the observatory and the 25th anniversary of Prof. De Sitters professorship, a committee composed of none less than Escher, Fokker, Hertzsprung, Hins, Oort, Pannekoek and Vissering, proposed to make a portrait of De Sitter. This portrait now hangs in the Oort Room. The letter¹⁾ proposing this plan has also been saved, coincidently found in an old yearbook.

The Leiden Observatory has of course, even after Kaiser, always had its financial problems. De Sitter was conscious of this. He wanted the very best for the smallest possible amount of money.

When the level, an aid to the Meridian telescope, could no longer be used because there was not enough ether left in the glass valve and as a result the air bubble stayed too big, the instrument maker and his assistant wanted to buy a new level. They first inquired about the cost of one. It would be fifty guilders. With this information (De Sitter always wanted all of the details in one hearing) they went to De Sitter to ask for approval to buy the new level. He looked at the level and repeated with a questioning intonation in his voice: “Fifty guilders? No, just repair that thing.” After much effort to fix the level, it was shown to De Sitter. He only smiled happily and nodded. The lesson was clear: don't waste money.

Willem de Sitter (1872-1934) was already a couple of years employed by the Sterewacht when he became director in 1919. He came from Groningen where he got his PhD with Kapteyn. He completely restructured the observatory and re-energized it. In his first yearly report he announced that a greater part of the transit circle observations of the last 40 years would be flushed trough the toilet, because of their relevance and time consuming processing. A quite astonishing and daring decision. de Sitter yearly report 1919 Dutch astronomers now could publish in the Bullitin of the Astonomical Institutes of the Netherlands. Three departments were installed: Theoretical astronomy, Meridian circle astrometry and Astrophysical astronomy. An agreement with the””Unie Sterrewag” (Johannesburg, S.A.) gave access to a much better an interesting site. De Sitter played an important role in the International Astronomical Union. Especially bringing together the former enemies in WW-I. Einstein who came regularly to Leyden to meet his physics friends (especially Ehrenfest and mentor Lorenz), also contacted de Sitter. Letter Lorentz to Einstein observation Eddington De Sitter was mainly a mathematician, understood the theory of relativity well and worked on cosmology.

His main work had been on the many body problem of Jupiter’s satellites. As cosmologist he became well known only many years after his death. Students did not come to Leyden for him, but for Hertzsprung. De Sitter mainly provided a context that boosted the institute to an unprecedented level as such.
Obituari de Sitter