Dutch mathematician, physicist, and astronomer.
The genealogy of the noted De Sitter family dates back to 1300. Many PhD academics and regents were born in it. His father was the judge Lamoraal Ulbo de Sitter (5 May 1846, Groningen – 6 September 1908, Arnhem). On 15 July 1871 he married in Assen, Catharina Theodore Wilhelmine Bertling (3 February 1846, Assen – 16 April 1909, Groningen). In 1871 he became substitute clerk (in Sneek), in 1879 Judge (in Assen) and in 1908 President of the court of Arnhem. Here a son Ernst Karel Johan de Sitter (22 November 1879, Arnhem – 19 September 1949, Haren, Groningen) was born. A sister, Wobbine Catharine de Sitter was born 8 August 1876 at 7h30 PM in Assen, when her father was a judge.
The parents of his mother were the judge Ernst Karel Johannes Bertling (19 January 1817, Veendam – 4 December 1889, Assen) and Wobbina Catharina van der Scheer (11 January 182, Coevorden – 27 April 1900, Arnhem). Catharina was first of six children. The parents of his father were Willem de Sitter (9 November 1820, Groningen – 13 January 1889, Groningen) and Wilhelmina Petronella Rengers (18 April 1823, Groningen – 16 November 1895, Groningen). They married 22 November 1843 in Groningen. Lamoraal Ulbo was the second of six children.
Willem de Sitter followed the Gymnasium in Arnhem. On 21 September 1891, Willem de Sitter studied mathematics and physics at the University of Groningen under Kapteyn and then joined the Groningen astronomical laboratory. He worked at the Cape Observatory in South Africa (1897–1899). Back in Leiden 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. From 1925-1926 he was Rector Magnificus in Leiden. From 1925 till 1928 he was President International Astronomical Union. In this function, he was Chairman of the 4th International Astronomical Congress in Leiden, that was opened at 5 July 1928 at 14:00 in the Ridderzaal of Den Haag.
De Sitter made major contributions to the field of physical cosmology. On 25 January 1932, he co-authored a paper with Albert Einstein in which they discussed the implications of cosmological data for the curvature of the universe (“On the relation between the expansion and the mean density of the universe” published on 15 March 1932 ).
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.
He received the Watson Gold Medal from the National Academy of Sciences in 1929, the Gold Medal of the Royal Astronomical Society and the Bruce Medal in 1931.
Personal
On 6 December 1898 De Sitter married Eleonore Suermondt (16 November 1870, Surabaya – 20 November 1952 at 2h30 PM, Leiden) in Cape Town, South Africa. He met her during his stay in South Africa. Her parents were Benjamin Alexander Suermondt (17 December 1837, Rotterdam – died who “door noodlottig toeval, door overstrooming” 28 April 1881 in Lebak Parei (Bantam) when she was young. Her mother was Henriette Louise van de Wiel (b 30 October 1854, died in Buitenzorg, Ind).
Willem and Eleonore Suermondt got five children.
Their first child Lamoraal Olbo de Sitter (Sept 1899, Cape Town – 10 January 1901 2 AM, Groningen) died early.
Their first daughter was Theodora de Sitter (28 September 1900 0h30 PM, Groningen – ?).
Their second son became the noted geologist Lamoraal Ulbo de Sitter (6 March 1902 5h30 PM , Groningen – 12 May 1980, Nistelrode). He married on 12 April 1926 in Jöngköping, Norway, Ingrid Clara Maria Frick (b ? Jöngköping – 12 December 1939 7 AM , Leiden), who died at age 39. Their son Lamoraal Ulbo de Sitter (2 April 1930, Jönköping – 18 December 2010, Heusden) became a noted Dutch sociologist and Professor of business administration at the Radboud university, specialising “veranderkunde” or in laws of chance (*). Their daughter Ragni de Sitter, born in Sanga Dalam died on 16 November 1934 3 AM in Tjepoe, Java at age 7.
His third son Aernout de Sitter (5 April 1905 6 PM, Groningen – 15 September 1944), became less noted astronomer in Indonesia, and died of beri-beri in Malay POW Camp. Aernout was the director of the Bosscha Observatory in Lembang, Indonesia,
His daughter Agnes de Sitter( 14 May 1908 12 AM, Groningen – ?), married on 11 September 1931 in Leiden, the poultry breeder Bernardus Solco Zoetelief Tromp (24 June 1904, Blaricum – ), a son of the painter Johannes Zoetelief Tromp (13 December 1872, Jakarta – 28 September 1947, Breteuil-sur-Iton) and Maria Bernardina Blommers (11 December 1876, Den Haag – before 1947, Breteuil sur Iton).
Willem de Sitter suffered the last years of his life from pulmonary disease. He died at age 62 on 20 November 1934 at 23h45 in Leiden of pneumonia.
Ten moon craters (. A,F,G,L,M,U,V,W,X)and the small planet 1686 De Sitter were named after him.
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