Willem de Sitter

Willem de Sitter (6 May 1872 – 20 November 1934) was a Dutch mathematician, physicist, and astronomer.

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 discussed the implications of cosmological data for the curvature of the universe. 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 1912 he became a member of the Royal Netherlands Academy of Arts and Sciences.

• James Craig Watson Medal (1929)
• Bruce Medal (1931)
• Gold Medal of the Royal Astronomical Society (1931)

• The crater De Sitter on the Moon
• Asteroid 1686 De Sitter

One of his sons, Ulbo de Sitter (1902 – 1980), was a Dutch geologist, and one of his sons was a Dutch sociologist Ulbo de Sitter (1930 – 2010).

Another son of Willem, Aernout de Sitter (1905 – 15 September 1944), was the director of the Bosscha Observatory in Lembang, Indonesia (then the Dutch East Indies), where he studied the Messier 4 globular cluster.

Einstein, Ehrenfest, Willem de Sitter, Eddington, and Lorentz in Leiden (1923)

• On the bearing of the Principle of Relativity on Gravitational Astronomy, 1911, Monthly Notices of the Royal Astronomical Society, Vol. 71, p. 388–415
• A proof of the constancy of the velocity of light, 1913, Proceedings of the Royal Netherlands Academy of Arts and Sciences, 1913, 15 II: 1297–1298
• Ein astronomischer Beweis für die Konstanz der Lichtgeschwindigkeit, 1913, Physikalische Zeitschrift, 14: 429
• On the constancy of the velocity of light, 1913, Proceedings of the Royal Netherlands Academy of Arts and Sciences, 1913, 16 I: 395-396
• Über die Genauigkeit, innerhalb welcher die Unabhängigkeit der Lichtgeschwindigkeit von der Bewegung der Quelle behauptet werden kann, 1913, Physikalische Zeitschrift, 14: 1267

• On Einstein's theory of gravitation and its astronomical consequences, Monthly Notices of the Royal Astronomical Society 76 (1916) 699-728; 77 (1916) 155-184; 78 (1917) 3-28