Willem de Sitter (May 6 1872 – November 20 1934) was a mathematician, physicist and astronomer. He was born in the city of Sneek in the Netherlands.

Willem de Sitter studied mathematics at Groningen University 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. From 1919 he was director of the Leiden Observatory.

De Sitter contributed to our concept of the universe. One of his accomplishments was to have co-authored a paper with Albert Einstein in 1932 in which they argued that there might be large amounts of matter which does not emit light, now commonly referred to as dark matter.

He won the James Craig Watson Medal in 1929 and the Bruce Medal in 1931, as well as the Gold Medal of the Royal Astronomical Society in that same year.

A crater on the Moon is named for him, as is the asteroid 1686 De Sitter .

See Also

• De Sitter universe
• De Sitter space
• De Sitter horizon

External links

• Bruce Medal page
• Awarding of Bruce Medal: PASP 43 (1931) 125
• Awarding of RAS gold medal: MNRAS 91 (1931) 422

Obituaries

• AN 253 (1934) 495/496 (one line)
• JRASC 29 (1935) 1
• MNRAS 95 (1935) 343
• Obs 58 (1935) 22
• PASP 46 (1934) 368 (one paragraph)
• PASP 47 (1935) 65