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Willem de Sitter was born on 6 May 1872 at Sneek in Friesland. He received his preparatory education at the Gymnasium at Arnhem, where his father was a judge, and then went to the University of Groningen. His intention was to become a mathematician, though he soon began to show an interest in physics and astronomy. While still a young student, he obtained permission to assist in some of the experiments carried out by Haga, the professor of physics, and he began to work in the Astronomical Laboratory at Groningen, where Kapteyn was engaged on the measurement of the plates of the Cape Photographic Survey. It was more or less as the result of a chance meeting with Sir David Gill that de Sitter definitely decided to become an astronomer. In 1896 October Gill visited Kapteyn; in his laboratory he found de Sitter at the measuring machine, measuring a plate. Gill had some conversation with him, and the following morning de Sitter, while having breakfast in his rooms, received a message that Gill wished to speak to 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. Gill invited de Sitter to come to the Cape as a computer and, as de Sitter afterwards stated in a letter to Gill, "thereby complete my astronomical education—or rather begin it, for up to that time I had never made a specialty of astronomy and intended to become a mathematician." De Sitter decided, after consulting with his parents, that he would first pass his examinations preparatory for the doctor's degree, and then go to the Cape. He reached the Cape in August 1897. The intention had been that he should make parallax observations with the McClean telescope, which had in 1894 been offered to the Cape Observatory by Mr. Frank McClean. But the telescope was not completed in time. De Sitter therefore assisted with the heliometer observations, determining the parallaxes of four stars and taking a part in the polar triangulation. He also occupied himself with a program of visual and photographic photometry. The purpose of these observations was to determine the difference in colour between stars near the Milky Way and stars near the galactic poles. The problem had arisen in connection with Kapteyn's work on the C.P.D., Kapteyn having found that the C.P.D. contained more stars in low galactic latitudes than the southern visual surveys and fewer in high latitudes. R. T. A. Innes, who had come to the Cape Observatory at Gill's invitation at the beginning of 1896 to take the post of secretary, assisted de Sitter in these observations and so began the friendship between de Sitter and Innes which was later to bear fruit in the scheme of cooperation between the Leiden Observatory and the Union Observatory in Johannesburg.
While de Sitter was at the Cape, Gill suggested to him that he should take up the reduction and discussion of a series of heliometer observations of Jupiter's satellites which had been made by Gill and Finlay in 1891, as an appropriate subject for his dissertation for the doctor's degree at the University of Groningen. These observations had been made as the result of a discussion some years earlier between Gill and J. C. Adams, on the subject of the kind of observations required improving the tables of the motions of Jupiter's satellites. Adams was at that time engaged in correcting Damoiseau's tables of the satellites. As a result of the discussion, Gill planned a series of accurate heliometer observations of the position angles and distances of the satellites with respect to each other. But pressure from other work delayed first the observations and then their reduction. De Sitter took up the subject with much interest; as a result of his discussion, strongly determined corrections of considerable amount to the inclinations and nodes of all the satellites and an accurate determination of the mass of Jupiter was obtained.
De Sitter left the Cape in December 1899, but the subject of Jupiter's satellites continued to engage his interest for the next thirty years. It was one in which his mathematical training could have full scope, but one which could not have been successfully attacked except by a person who, like de Sitter, had a critical appreciation of the value and limitations of observations. The training at the Cape under Gill, who possessed a very high degree of insight into the value of observations and the errors to which they are liable, was undoubtedly to a large extent responsible for the development in de Sitter of the same insight.
At de Sitter's request, several series of photographic observations of the satellites of Jupiter were obtained at Greenwich, the Cape, Johannesburg, Leiden, and Pulkovo. These were reduced under de Sitter's supervision and critically discussed by him. In the mathematical investigation, he used an intermediate orbit in which the eccentricities were taken as zero and the great inequalities appeared as equations of the centre. For the three inner satellites this gave a better approximation to the true motion than the undisturbed Keplerian motion. Old observations of eclipses of the satellites, extending back to the year 1668, were also reduced and discussed. It was not until 1929 that de Sitter was able to publish the definitive values of the orbital elements and masses of the four satellites. The theoretical discussion was published in three parts, in 1918, 1919 and 1925; the second part was carried out by Dr. A. J. Leckie, under de Sitter's general direction. Tables of the motions of the satellites, based on the new theory, have not yet been published, but much preparatory work has been carried out and it is to be hoped that it will be possible for one of de Sitter's collaborators to complete the reduction to the form of tables.
Meanwhile, upon the retirement of Professor H. G. v. d. Sande Bakhuyzen, his office was divided into two parts. One, the Directorship of the Leiden Observatory with a Professorship of Practical Astronomy, was filled by E. F. v. d. Sande Bakhuyzen. The other, the Chair of Astronomy in the Leiden Observatory, was filled by de Sitter in 1908. On the death of E. F. v. d. Sande Bakhuyzen in 1918, de Sitter was also appointed Director of the Leiden Observatory. This old-established institution, whose tercentenary was recently celebrated, had ceased to be an institution of importance. The government had decided on a complete reorganization and considerable enlargement. De Sitter and Kapteyn together drew up plans for a reorganization along the lines of modern developments. The observatory had heretofore been concerned mainly with fundamental observations; astrophysical and theoretical departments were now formed, the fundamental department was reorganised and the observatory became one of the most important in Europe. In connection with the fundamental work, de Sitter planned a programme for the determination of fundamental declinations based on observations of azimuth from a station near the equator and from two stations north and south of the equator, at a latitude of about 30°. A special instrument, largely of de Sitter's design, was constructed for these observations by Messrs. Cooke, Troughton & Simms, Ltd., of York, but the completion of the programme of observations was delayed by financial difficulties and at the present time only one series of observations, at a station in Kenya near the equator, has been obtained.
The reorganization of the observatory and the considerable amount of administrative work entailed in its direction made considerable demands on de Sitter's time. Yet papers of importance on various topics continued to appear with remarkable regularity. This is all the more surprising as he was far from robust, and he suffered from a serious illness for some years, which for a time necessitated his residence in Switzerland. In 1915 appeared the first of a series of papers on the coordination of astronomical constants. Newcomb's Fundamental Constants of Astronomy was published in 1895, and the large mass of observations accumulated since that date permitted various improvements to be made. No one was better suited than de Sitter to undertake such a revision. The first paper was concerned mainly with the figure and composition of the Earth; in 1927 a second paper appeared dealing with other constants such as precession, nutation, solar parallax, lunar parallax, and the mass of the Moon. A new investigation into the continuance of this work had almost been completed at the time of his death.
An important series of investigations by de Sitter concerned the theory of relativity. In 1911, he contributed to the Monthly Notices a paper dealing with the small deviations in the motions of the Moon and planets from Newtonian dynamics to be expected on the basis of the restricted principle of relativity. Shortly after the appearance of Einstein's paper on the generalized theory, de Sitter communicated a series of three papers to the Royal Astronomical Society in 1916-17, giving not only a complete exposition of the mathematical theory but also a detailed examination of its astronomical consequences. The third of these papers contains his most important contribution to the subject, for in what is now generally called the "de Sitter universe" was put forward as an alternative to the "Einstein universe." These papers appeared in the Monthly Notices at a time when, owing to war conditions, Einstein's paper was practically unobtainable. Their appearance not only stimulated the study of the new theory in this country but also considerably influenced the form in which the theory was presented The British eclipse expeditions of 1919, which provided the first evidence in support of Einstein's conclusions as to the amount of deviation of rays of light in passing near the Sun, would probably not have been sent out had de Sitter's papers not appeared. The confirmation of the deflection, provided by the data derived from these expeditions, did much to secure general acceptance of the theory at a time when many scientific men had hesitated to accept it. More recently, after it had been proved that the universe represented by the static solution of the theory was unstable, de Sitter returned again to the theory and examined mathematically the various types of non-static solution possible; he showed that there were three types of solution. These he termed the expanding universe of the first kind, in which the radius decreases to a small value and then expansion to an infinite radius follows; the expanding universe of the second kind, in which there is continuous expansion from an initial finite radius; and the oscillating universe in which the radius alternately increases and decreases.
Another subject that attracted de Sitter's attention was the variability of the Earth's rotation. He was led to consider this matter as a result of his discussion of the early observations of the eclipses of Jupiter's satellites. He used these as a check on the uniformity of astronomical time, as measured by the rotation of the Earth. His discussion was made with characteristic thoroughness and left no doubt as to the reality of the variations in the rate of rotation, and that these variations may occur with considerable abruptness. He concluded that all the observed phenomena could be explained by supposing that there is a varying tidal friction which influences the rotation of the Earth and also the motion of the Moon and that, in addition, there are comparatively sudden changes in the moment of inertia of the Earth, which affect only the Earth's rotation
Mention has been made of the cooperation between the Union and Leiden Observatories. The observing conditions at Leiden are very poor, whereas at the Union Observatory, Johannesburg, the amount of clear sky is so great that the instruments cannot be operated to their full efficiency by the limited staff. Through his friendship with Innes, de Sitter came to an arrangement by which the Leiden Observatory stations an observer permanently in Johannesburg to use one of the telescopes. This cooperation has proved very fruitful in results. Further developments were planned; through a large gift from the Rockefeller Foundation, a powerful instrument was obtained, which is to be erected by the Leiden Observatory at the Union Observatory, Johannesburg. This instrument, constructed by Messrs. Sir Howard Grubb, Parsons & Co., was almost completed at the time of de Sitter's death
The profound effect on de Sitter's life of the chance meeting with Gill has been emphasized. De Sitter's visit to the Cape had a further result; it was there he first met the lady who became his wife. Their married life was singularly happy; they had two sons and two daughters, all of whom are married. De Sitter's greatest joy was to have his children and his grandchildren around him in the charming home at the observatory.
De Sitter was a frequent visitor to England, and his visits were always welcomed by his colleagues. He was a not infrequent visitor to the meetings of the British Association and attended the two meetings of the Association in South Africa in 1905 and 1929. He was elected an Associate of the Society in 1909 and was awarded its Gold Medal in 1931. He was the recipient of the Watson Gold Medal of the National Academy of Sciences, Washington, in 1929, and of the Bruce Gold Medal of the Astronomical Society of the Pacific in 1931. In the period 1925-28 he was President of the International Astronomical Union. He died on November 19 last from pneumonia at the comparatively early age of sixty-two years, and at a time when there was no apparent diminution in his output of work. The loss, not only to his observatory but to the whole world of astronomy, is severe.
H. S. J
While de Sitter was at the Cape, Gill suggested to him that he should take up the reduction and discussion of a series of heliometer observations of Jupiter's satellites which had been made by Gill and Finlay in 1891, as an appropriate subject for his dissertation for the doctor's degree at the University of Groningen. These observations had been made as the result of a discussion some years earlier between Gill and J. C. Adams, on the subject of the kind of observations required improving the tables of the motions of Jupiter's satellites. Adams was at that time engaged in correcting Damoiseau's tables of the satellites. As a result of the discussion, Gill planned a series of accurate heliometer observations of the position angles and distances of the satellites with respect to each other. But pressure from other work delayed first the observations and then their reduction. De Sitter took up the subject with much interest; as a result of his discussion, strongly determined corrections of considerable amount to the inclinations and nodes of all the satellites and an accurate determination of the mass of Jupiter was obtained.
De Sitter left the Cape in December 1899, but the subject of Jupiter's satellites continued to engage his interest for the next thirty years. It was one in which his mathematical training could have full scope, but one which could not have been successfully attacked except by a person who, like de Sitter, had a critical appreciation of the value and limitations of observations. The training at the Cape under Gill, who possessed a very high degree of insight into the value of observations and the errors to which they are liable, was undoubtedly to a large extent responsible for the development in de Sitter of the same insight.
At de Sitter's request, several series of photographic observations of the satellites of Jupiter were obtained at Greenwich, the Cape, Johannesburg, Leiden, and Pulkovo. These were reduced under de Sitter's supervision and critically discussed by him. In the mathematical investigation, he used an intermediate orbit in which the eccentricities were taken as zero and the great inequalities appeared as equations of the centre. For the three inner satellites this gave a better approximation to the true motion than the undisturbed Keplerian motion. Old observations of eclipses of the satellites, extending back to the year 1668, were also reduced and discussed. It was not until 1929 that de Sitter was able to publish the definitive values of the orbital elements and masses of the four satellites. The theoretical discussion was published in three parts, in 1918, 1919 and 1925; the second part was carried out by Dr. A. J. Leckie, under de Sitter's general direction. Tables of the motions of the satellites, based on the new theory, have not yet been published, but much preparatory work has been carried out and it is to be hoped that it will be possible for one of de Sitter's collaborators to complete the reduction to the form of tables.
Meanwhile, upon the retirement of Professor H. G. v. d. Sande Bakhuyzen, his office was divided into two parts. One, the Directorship of the Leiden Observatory with a Professorship of Practical Astronomy, was filled by E. F. v. d. Sande Bakhuyzen. The other, the Chair of Astronomy in the Leiden Observatory, was filled by de Sitter in 1908. On the death of E. F. v. d. Sande Bakhuyzen in 1918, de Sitter was also appointed Director of the Leiden Observatory. This old-established institution, whose tercentenary was recently celebrated, had ceased to be an institution of importance. The government had decided on a complete reorganization and considerable enlargement. De Sitter and Kapteyn together drew up plans for a reorganization along the lines of modern developments. The observatory had heretofore been concerned mainly with fundamental observations; astrophysical and theoretical departments were now formed, the fundamental department was reorganised and the observatory became one of the most important in Europe. In connection with the fundamental work, de Sitter planned a programme for the determination of fundamental declinations based on observations of azimuth from a station near the equator and from two stations north and south of the equator, at a latitude of about 30°. A special instrument, largely of de Sitter's design, was constructed for these observations by Messrs. Cooke, Troughton & Simms, Ltd., of York, but the completion of the programme of observations was delayed by financial difficulties and at the present time only one series of observations, at a station in Kenya near the equator, has been obtained.
The reorganization of the observatory and the considerable amount of administrative work entailed in its direction made considerable demands on de Sitter's time. Yet papers of importance on various topics continued to appear with remarkable regularity. This is all the more surprising as he was far from robust, and he suffered from a serious illness for some years, which for a time necessitated his residence in Switzerland. In 1915 appeared the first of a series of papers on the coordination of astronomical constants. Newcomb's Fundamental Constants of Astronomy was published in 1895, and the large mass of observations accumulated since that date permitted various improvements to be made. No one was better suited than de Sitter to undertake such a revision. The first paper was concerned mainly with the figure and composition of the Earth; in 1927 a second paper appeared dealing with other constants such as precession, nutation, solar parallax, lunar parallax, and the mass of the Moon. A new investigation into the continuance of this work had almost been completed at the time of his death.
An important series of investigations by de Sitter concerned the theory of relativity. In 1911, he contributed to the Monthly Notices a paper dealing with the small deviations in the motions of the Moon and planets from Newtonian dynamics to be expected on the basis of the restricted principle of relativity. Shortly after the appearance of Einstein's paper on the generalized theory, de Sitter communicated a series of three papers to the Royal Astronomical Society in 1916-17, giving not only a complete exposition of the mathematical theory but also a detailed examination of its astronomical consequences. The third of these papers contains his most important contribution to the subject, for in what is now generally called the "de Sitter universe" was put forward as an alternative to the "Einstein universe." These papers appeared in the Monthly Notices at a time when, owing to war conditions, Einstein's paper was practically unobtainable. Their appearance not only stimulated the study of the new theory in this country but also considerably influenced the form in which the theory was presented The British eclipse expeditions of 1919, which provided the first evidence in support of Einstein's conclusions as to the amount of deviation of rays of light in passing near the Sun, would probably not have been sent out had de Sitter's papers not appeared. The confirmation of the deflection, provided by the data derived from these expeditions, did much to secure general acceptance of the theory at a time when many scientific men had hesitated to accept it. More recently, after it had been proved that the universe represented by the static solution of the theory was unstable, de Sitter returned again to the theory and examined mathematically the various types of non-static solution possible; he showed that there were three types of solution. These he termed the expanding universe of the first kind, in which the radius decreases to a small value and then expansion to an infinite radius follows; the expanding universe of the second kind, in which there is continuous expansion from an initial finite radius; and the oscillating universe in which the radius alternately increases and decreases.
Another subject that attracted de Sitter's attention was the variability of the Earth's rotation. He was led to consider this matter as a result of his discussion of the early observations of the eclipses of Jupiter's satellites. He used these as a check on the uniformity of astronomical time, as measured by the rotation of the Earth. His discussion was made with characteristic thoroughness and left no doubt as to the reality of the variations in the rate of rotation, and that these variations may occur with considerable abruptness. He concluded that all the observed phenomena could be explained by supposing that there is a varying tidal friction which influences the rotation of the Earth and also the motion of the Moon and that, in addition, there are comparatively sudden changes in the moment of inertia of the Earth, which affect only the Earth's rotation
Mention has been made of the cooperation between the Union and Leiden Observatories. The observing conditions at Leiden are very poor, whereas at the Union Observatory, Johannesburg, the amount of clear sky is so great that the instruments cannot be operated to their full efficiency by the limited staff. Through his friendship with Innes, de Sitter came to an arrangement by which the Leiden Observatory stations an observer permanently in Johannesburg to use one of the telescopes. This cooperation has proved very fruitful in results. Further developments were planned; through a large gift from the Rockefeller Foundation, a powerful instrument was obtained, which is to be erected by the Leiden Observatory at the Union Observatory, Johannesburg. This instrument, constructed by Messrs. Sir Howard Grubb, Parsons & Co., was almost completed at the time of de Sitter's death
The profound effect on de Sitter's life of the chance meeting with Gill has been emphasized. De Sitter's visit to the Cape had a further result; it was there he first met the lady who became his wife. Their married life was singularly happy; they had two sons and two daughters, all of whom are married. De Sitter's greatest joy was to have his children and his grandchildren around him in the charming home at the observatory.
De Sitter was a frequent visitor to England, and his visits were always welcomed by his colleagues. He was a not infrequent visitor to the meetings of the British Association and attended the two meetings of the Association in South Africa in 1905 and 1929. He was elected an Associate of the Society in 1909 and was awarded its Gold Medal in 1931. He was the recipient of the Watson Gold Medal of the National Academy of Sciences, Washington, in 1929, and of the Bruce Gold Medal of the Astronomical Society of the Pacific in 1931. In the period 1925-28 he was President of the International Astronomical Union. He died on November 19 last from pneumonia at the comparatively early age of sixty-two years, and at a time when there was no apparent diminution in his output of work. The loss, not only to his observatory but to the whole world of astronomy, is severe.
H. S. J
Willem de Sitter's obituary appeared in Journal of the Royal Astronomical Society 95:4 (1935), 343-347.