**Willem de Sitter** was a Dutch mathematician, physicist, and astronomer who made major contributions in physical cosmology.

- Willem attended secondary school in Arnhem, studying at the Arnhem Gymnasium.
- 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.
- The turning point in de Sitter's life came in 1896 when David Gill visited Kapteyn to discuss progress on the southern sky survey.
- the following morning de Sitter, while having breakfast in his rooms, received a message that Gill wished to speak with him in the laboratory.
- De Sitter did not possess at that time the fluent command of the English language which he afterwards acquired; Kapteyn was lecturing at the time and Mrs Kapteyn acted as interpreter at the interview.
- He received a Bachelor's Degree in 1897 and left for Cape Town in the summer of that year arriving in August.
- 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.
- After de Sitter returned to Groningen in 1899 he was appointed as an assistant in the Astronomical Laboratory and also continued to work towards his doctorate, advised by Kapteyn.
- Eleonora was born in Surabaya, the second largest city in the Dutch East Indies, and she had met de Sitter in the Cape where she was working as a school teacher.
- Lamoraal Ulbo de Sitter became well-known as a structural geologist and died in Nistelrode in 1980.
- Aernout de Sitter served as director of the Bosscha Observatory in Lembang, Dutch East Indies.
- After the Japanese invasion of the Dutch East Indies he was arrested and put into a detention camp where he died on 5 September 1944.
- Hendricus Gerardus van de Sande Bakhuyzen had been Professor of Astronomy and Director of the University Observatory at the University of Leiden from his appointment in 1872.
- On taking up the chair, de Sitter gave his inaugural lecture on The New Methods in Celestial Mechanics.
- E F van de Sande Bakhuyzen died in 1918 and in the following year de Sitter was appointed Director of the Leiden Observatory in addition to his professorship.
- He undertook a complete reorganisation 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 centres in the world.
- In looking through a number of 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 reorganise almost entirely, nor his constant activities in many matters connected with the University could slow up the flow of his scientific investigations.
- In 1913 de Sitter produced an argument based on observations 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.
- De Sitter corresponded with Paul Ehrenfest in 1916, and he proposed that a four-dimensional space-time would fit in with cosmological models based on general relativity.
- This was significant since Mach had stated a principle that local inertial frames of reference were determined by the large-scale distribution of mass in the universe.
- De Sitter's work led directly to Arthur Eddington's 1919 expedition to measure the gravitational deflection of light rays passing near the Sun, results which, at that time, could only be obtained during an eclipse.
- De Sitter, unlike Einstein, maintained that relativity actually implied that the universe was expanding, theoretical results which were later verified observationally and only then accepted by Einstein.
- Einstein had introduced the cosmological constant in 1917 to solve a significant problem concerning the universe, which had also troubled Newton before him, namely why does the universe not collapse under gravitational attraction.
- detracts from the symmetry and elegance of Einstein's original theory, one of whose chief attractions was that it explained so much without introducing any new hypothesis or empirical constant.
- In the 1990s observational evidence suggested that the expansion of the universe is accelerating.
- One way to incorporate this into the relativistic model is to reintroduce the cosmological constant.
- In 1932 Einstein and de Sitter published a joint paper in which they proposed the Einstein-de Sitter model of the universe.
- They argued in this paper that there might be large amounts of matter which does not emit light and has not been detected.
- However the dark matter postulated by Einstein and de Sitter in 1932 still remains a mystery in that its nature is still unknown but is the subject of major research efforts today.
- Although de Sitter is best known for this work on relativity, he made many other contributions of great significance.
- 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.
- 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.
- Immediately de Sitter had another topic for which he had precisely the right skills to make a breakthrough.
- His study showed that there is varying tidal friction which affects both the earth and the moon and, in addition, sudden changes which occur in the moment of inertia of the earth.
- Another study which de Sitter undertook was to refine the data for the fundamental constants of astronomy.
- Simon Newcomb had published values for these constants in 1895 and in a rather remarkable international agreement in Paris in 1896, it had been decided that the ephemerides of every country in the world should use Newcomb's values for these constants.
- In 1915 de Sitter published his first paper on improving the values, this one being concerned almost entirely with the figure and composition of the earth.
- We have, of course, the set of constants officially adopted in the national ephemerides ...
- His second paper on the fundamental constants was published in 1927 and dealt with the constants associated with precession, nutation, solar parallax, lunar parallax and the mass of the moon.
- At the time of his death, de Sitter had almost completed a new updating of these constants.
- Many honours were given to de Sitter for his outstanding contributions.
- He received the Watson Gold Medal from the National Academy of Sciences in 1929.
- His intellectual abilities cover so wide a range and penetrate so deeply and so minutely into practical astronomy and the mathematical theories to explain what is observed, that only an intensive study of his brilliant work could do justice to the greatness of the man.
- In 1931 de Sitter received the Bruce Medal from the Astronomical Society of the Pacific.
- He was elected as President of the International Astronomical Union and served in this capacity from 1925 to 1928.
- De Sitter suffered from chest complaints for several years but seemed to overcome them.
- He is not a cold, dispassionate juggler of Greek letters, a balancer of equations, but rather an artist in whom wild flights of the imagination are restrained by the formalism of a symbolic language and the evidence of observation.
- Here we have something of the direct personal experience of the outer world, of the significance of nature's wonders, that comes only to a Beethoven or a Milton.
- The expanding universe of de Sitter must be regarded as something more than an inexorable conclusion drawn from the strictest kind of logic with which the human mind is familiar.

Born 6 May 1872, Sneek, Netherlands. Died 20 November 1934, Leiden, Netherlands.

View full biography at MacTutor

Astronomy, Origin Netherlands

**Oâ€™Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive