Willem de Sitter, Dutch astronomer. Author unknown photographer Wide World Photos. Wikimedia Commons

Top 10 Interesting Facts about Willem de Sitter


 

Willem de Sitter was a Dutch mathematician, physicist, and astronomer who made major contributions in physical cosmology. Born in Sneek, he studied mathematics at the University of Groningen and then joined the Groningen astronomical laboratory. He also worked at the Cape Observatory in South Africa (1897–1899). In 1908, de Sitter was appointed to the chair of astronomy at Leiden University. He was also director of the Leiden Observatory from 1919 until his death in 1934.

1. He Co-Authored A Paper With Albert Einstein

He is known to have made major contributions to the field of physical cosmology. He co-authored a paper with Albert Einstein in 1932. In this paper they discussed the implications of cosmological data for the curvature of the universe. He is also credited with the concept of the de Sitter space and de Sitter universe. He emphasizes on general relativity in which there is no matter and a positive cosmological constant. This results in an exponentially expanding, empty universe.

2. His Defied The Families’ Tradition Of Studying Law

Willem de Sitter’s came from a family of lawyers. His father was a judge and therefore expected his son to follow that tradition. However, he took a different career path with his love of mathematics and science.

After graduating from the school, he entered the University of Groningen with the intention of taking a mathematics degree. In addition to his love for mathematics, his enjoyment of physics and carrying out physical experiments grew during his undergraduate years. He obtained permission to assist with experiments being carried out by Hermanus Haga, the professor of physics at the University of Groningen, and as a consequence began to work in the Astronomical Laboratory at Groningen.

3. His Passion Changed From Mathematics To Astronomy

The turning point in de Sitter’s life came in 1896 when David Gill visited Kapteyn to discuss progress on the southern sky survey. He was invited to complete his astronomical education though initially his interest was to become a mathematician.

De Sitter consulted his parents and decided to take their advice and complete his examinations at Groningen before taking up Gill’s offer. He received a bachelor’s degree in 1897 and left for Cape Town in the summer of that year arriving in August. He worked at the Cape Observatory in South Africa for two years taking part in photometric and heliometer programmes.

It was at Gill’s suggestion that de Sitter also worked towards his doctorate studying heliometer observations of Jupiter’s moons which had been made by Gill in 1891. He was later appointed as an assistant in the Astronomical Laboratory and also continued to work towards his doctorate. He submitted his thesis Discussion of Heliometer Observations of Jupiter’s Satellites to the University of Groningen in 1901. Jan Hendrik Oort writes in     

4. He Led One Of The Leading Astronomical Centers In The World

In 1918, de Sitter was appointed Director of the Leiden Observatory in addition to his professorship. He undertook a complete reorganization of astronomy at Leiden, dividing it into three divisions: Fundamental Astronomy of position or astrometry; Astrophysics; and Celestial Mechanics or theoretical astronomy. Under his leadership it was one of the leading astronomical centers in the world.

In looking through several his more important publications, one is struck with the rapidity with which discussions on different topics follow each other and by the amount of thorough critical thinking of which each of these brings evidence. Neither a serious illness from which he suffered during some years, nor the directorship of an observatory which he undertook to reorganize almost entirely, nor his constant activities in many matters connected with the University could slow up the flow of his scientific investigations.

5. De Sitter Believed That The Velocity Of Light Was Independent Of The Velocity Of The Source

In 1919 Sitter came up with the theory of double star systems which proved that the velocity of light was independent of the velocity of the source. It put to rest attempts which had been made up until this time to find emission theories of light which depended on the velocity of the source but were not in conflict with experimental evidence.

6. He Contributed To The Einstein-De Sitter Model Of The Universe

In 1932 Einstein and de Sitter published a joint paper in which they proposed the Einstein-de Sitter model of the universe. This is a particularly simple solution of the field equations of general relativity for an expanding universe. They argued in this paper that there might be large amounts of matter which does not emit light and has not been detected. This matter, now called ‘dark matter’, has since been shown to exist by observing is gravitational effects.

 However, the dark matter postulated by Einstein and de Sitter in 1932 remains a mystery in that its nature is still unknown but is the subject of major research efforts today.

7. His Doctorate Was On The Satellites Of Jupiter

Although de Sitter is best known for this work on relativity, he made many other contributions of great significance. His doctorate had been on the satellites of Jupiter, and he maintained an interest in this topic throughout his life. He used data on eclipses of the satellites dating back to 1668 to produce definitive data on the orbital elements and masses of the four satellites.

He published other theoretical discussion in three papers of 1918, 1919 and 1925. Finally in 1929 he produced his definitive results, but he was still working on tables of the motions of the satellites when he died in 1934. During his investigations of the timings of the eclipses of Jupiter’s moons, he realized that his data was being affected by variations in the speed of rotation of the earth.

8. He Received Many Honors For His Outstanding Contributions

Many honors were given to de Sitter for his outstanding contributions. He received the Watson Gold Medal from the National Academy of Sciences in 1929. The Medal was presented to him in Washington, USA, by Armin Otto Leuschner, Professor of Astronomy of the University of California and Chairman of the Trustees of the Watson Fund.

In 1931 de Sitter received the Bruce Medal from the Astronomical Society of the Pacific. Also, in 1931 he received the Gold Medal of the Royal Astronomical Society. This was in recognition to his theoretical investigations on the orbits of the satellites of Jupiter, and for his contribution to the Theory of Relativity.

9.  He Was A Member Of The Royal Netherlands Academy Of Sciences

In 1912, he became a member of the Royal Netherlands Academy of Arts and Sciences.

10. He Served As President Of The International Astronomical Union

He was elected as President of the International Astronomical Union and served in this capacity from 1925 to 1928.