As a boy Kapteyn showed outstanding intellectual ability and curiosity. At the age of sixteen he passed the entrance examination for the University of Utrecht, which, however, his parents judged him too young to enter until the following year. He studied mathematics and physics and obtained the doctor’s degree with a thesis on the vibration of a membrane.
Career
Gallery of Jacobus Kapteyn
1908
Mount Wilson Observatory, Pasadena, California, United States
Photo of Kapteyn made at the Mount Wilson Observatory in 1908.
Gallery of Jacobus Kapteyn
1909
Mount Wilson Observatory, Pasadena, California, United States
Jacobus Kapteyn with his wife Catharina Elisabeth Kalshoven in front of their tent at the Mount Wilson Observatory, Pasadena, California. They visited this place from July to October 1909 for his astronomical work.
Gallery of Jacobus Kapteyn
Portrait of Jacobus Kapteyn painted by Jan Pieter Veth.
Gallery of Jacobus Kapteyn
Photo of Jacobus Kapteyn.
Gallery of Jacobus Kapteyn
Photo of Jacobus Kapteyn.
Gallery of Jacobus Kapteyn
Painting of Jacobus Kapteyn at his desk.
Gallery of Jacobus Kapteyn
Photo of young Jacobus Kapteyn.
Achievements
The crater on the Moon was named after Kapteyn.
Membership
Royal Netherlands Academy of Arts and Sciences
Royal Netherlands Academy of Arts and Sciences, Amsterdam, Netherlands
In 1888 Kapteyn became a member of the Royal Netherlands Academy of Arts and Sciences.
Royal Society
Royal Society, London, England
Kapteyn was a foreign member of the Royal Society.
National Academy of Sciences
National Academy of Sciences, Washington, District of Columbia, United States
Kapteyn was a foreign associate of the National Academy of Sciences.
Saint Petersburg Academy of Sciences
Saint Petersburg Academy of Sciences, Saint Petersburg, Russia
Kapteyn was a corresponding member of the Saint Petersburg Academy of Sciences.
Royal Society of Edinburgh
Royal Society of Edinburgh, Edinburgh, Scotland
Kapteyn was an honorary fellow of the Royal Society of Edinburgh.
Awards
Gold Medal of the Royal Astronomical Society
1902
Kapteyn was awarded the Gold Medal of the Royal Astronomical Society in 1902.
James Craig Watson Medal
1913
Kapteyn was awarded the James Craig Watson Medal in 1913.
Mount Wilson Observatory, Pasadena, California, United States
Jacobus Kapteyn with his wife Catharina Elisabeth Kalshoven in front of their tent at the Mount Wilson Observatory, Pasadena, California. They visited this place from July to October 1909 for his astronomical work.
As a boy Kapteyn showed outstanding intellectual ability and curiosity. At the age of sixteen he passed the entrance examination for the University of Utrecht, which, however, his parents judged him too young to enter until the following year. He studied mathematics and physics and obtained the doctor’s degree with a thesis on the vibration of a membrane.
Jacobus Cornelis Kapteyn was a Dutch astronomer who carried out extensive studies of the Milky Way and used photography and statistical methods in determining the motions and distribution of stars. He also served as a professor of astronomy and theoretical mechanics at the University of Groningen.
Background
Jacobus Cornelis Kapteyn was born on January 19, 1851, in Barneveld, Netherlands. He was the ninth of fifteen children of Gerrit J. Kapteyn and Elisabeth C. Koomans, who conducted a boarding school for boys in Barneveld. Many of these children were extraordinarily gifted in science.
Education
As a boy, Kapteyn showed outstanding intellectual ability and curiosity. At the age of sixteen he passed the entrance examination for the University of Utrecht, which, however, his parents judged him too young to enter until the following year. He studied mathematics and physics and obtained the doctor’s degree with a thesis on the vibration of a membrane.
Kapteyn was interested in many branches of science, and it was mostly through his accepting (1875) a position as observator at the Leiden Observatory that he began his career in astronomy. In 1878, at the age of twenty-seven, he was elected to the newly instituted chair of astronomy and theoretical mechanics at the University of Groningen, which he held until his retirement in 1921 at the age of seventy.
At the time of studies in the field of stellar astronomy, the problem of the space distribution of the stars was still tantamount to the problem of the structure of the universe. Kapteyn’s work presents the first major step in this field after the great works of William and John Herschel. It culminated in the views presented in the article “First Attempt at a Theory of the Arrangement and Motion of the Sidereal System,” published in the May 1922 issue of the Astrophysical Journal, shortly before Kapteyn’s death. Seeking to resolve the structure and kinematic properties of the stellar system, Kapteyn devoted his efforts both to the problem of the methods to be developed and their mathematical aspects, and to that of obtaining proper observational data. For the latter purpose he established and participated in extensive international observational projects of many kinds. These have served astronomical research in fields remote from Kapteyn’s own.
Kapteyn’s first major achievement was the compilation, together with David Gill, of the Cape Photographic Durchmusterung, a catalogue of stars in the southern hemisphere. Their approach to this project was quite untraditional according to astronomical practice of that time. Since the University of Groningen, in spite of Kapteyn’s requests, could not provide him with a telescope, he looked for other ways to contribute to the observational work. In 1885 he contacted Gill, then director of the Royal Observatory in Cape Town, South Africa, to offer to measure the photographic plates, covering the whole southern sky, which Gill had taken at the Cape with a Dallmeyer objective. These would be measurements of the position and apparent brightness of the stars, down to magnitudes comparable with those of the Bonner Durchmusterung, and thus would supplement, by photographic means, what had been accomplished by Argelander, by visual means, for the northern sky.
The extremely laborious work was finished after thirteen years of excellent collaboration. For the measurement of the plates, done from 1886 to 1896, Kapteyn devised an unconventional method. Instead of measuring x and y coordinates, he used a theodolite, observing the plate from a distance equal to the focal length of the telescope that had produced the plates. The catalogue was published as volumes III, IV, and V of the Annals of the Royal Observatory, Cape Town (1896-1900) and lists the positions of 454,875 stars, between the South Pole and declination -18°, down to the tenth magnitude. An invaluable reference work on the southern sky, it is remarkably free of errors because of the painstaking care with which Kapteyn himself participated in much of the routine work. Assistance in the routine included labor by certain convicts of the stale prison at Groningen, who were put at Kapteyn’s disposal by the prison authorities.
The measurements were all carried out in two small rooms of the physiological laboratory of the University of Groningen. Thus started Kapteyn’s “astronomical laboratory,” a kind of institute unique at that time, which soon would become world famous and recognized a much-needed complement to institutes equipped with telescopes. After being housed in various other “guest” institutes, the “laboratory” in 1913 acquired the whole of the building of the original physiological laboratory.
When tests of the method using /x as a distance indicator gave unsatisfactory results, Kapteyn found that the assumption of random motion was incorrect: preferred directions did exist. It appeared that the stars belong to two different, but intermingled, groups having different mean motions with respect to the sun. This phenomenon, termed “the two star streams,” was announced by Kapteyn at the International Congress of Science at St. Louis in 1904 and before the British Association in Cape Town in 1905 and deeply impressed the astronomical world. It demonstrated that a certain order, rather than the hitherto assumed random motion, dominated stellar motions.
In order to make sure that selection from the fainter stars would involve as much as possible the same stars for each observational program, Kapteyn devised a plan which was proposed to the international astronomical world in the booklet Plan of Selected Areas. His plan resulted from many letters and discussions with colleagues abroad. The Plan proposed to concentrate work on 206 stellar areas, uniformly demarcated over the sky and at declinations +90°, +-75°, +60° to -90°. Photographic and visual magnitudes were to be measured for all stars in these areas (±200,000); and, for more limited numbers, the quantities more difficult to measure, such as proper motion, parallax, spectral type, and radial velocity. This observational material would provide a proper sampling of the stellar system for the purpose of revealing its main structural features. At the instigation of the astronomer E. C. Pickering, a supplementary program of forty-six areas was proposed, chosen where the Milky Way shows particularly striking features, such as excessive star density and dark or bright nebulae. Pickering’s program became known as the “Special Plan.” Methods for observing and for evaluation of the material and the prospects for analyses were extensively discussed in the booklet.
Astronomical institutes throughout the world responded most favorably to the proposal - not least because of the cooperative spirit Kapteyn and his laboratory had shown on many occasions when their help was solicited by others. Work on Kapteyn’s plan, and to a lesser degree on Pickering’s special plan, progressed during the first half of the twentieth century and continues to be an outstanding example of international scientific collaboration. To date forty-three observatories have in one way or other collaborated. Shortly after international agreement on the plan had been reached, its supervision was placed in the hands of an international committee of prominent astronomers; W. S. Adams, F. W. Dyson, Gill, Hale, Kustner, E. C. Pickering, K. Schwarzschild, and Kapteyn himself. The committee was later incorporated into the International Astronomical Union as one of its commissions, and progress reports on the plan are to be found in the Transactions of the union.
With the newly obtained results on stellar density distribution and the new knowledge of stellar kinematics (the peculiar motions, solar motion, and star streams), Kapteyn toward the end of his career developed a dynamical theory of the system. Such a theory aimed at explaining both observed density distribution and motions in terms of gravitational forces, and it would do this on the assumption that the system is in a state of equilibrium.
Apart from these main achievements, Kapteyn made essential contributions in many other fields. Among these are his attempts, in his early years at the Leiden observatory, to improve upon the measurement of trigonometric parallaxes with the meridian telescope and his later efforts to apply photographic methods for this purpose as well as for the measurement of stellar magnitudes. In his early years he also devised a method to find the altitude of the equatorial pole which would be free of errors in the declinations of the stars and would be independent of errors in the atmospheric refraction. In the course of his researches he introduced many concepts that have come into common acceptance in astronomy, including those of absolute magnitude and color index.
(A photo of the instrument of Kapteyn (and its accompanyin...)
Politics
Kapteyn had a keen sense of justice and suffered deeply when World War I disrupted the international communication between scientists. He firmly believed in the duty of scientists to bridge the gaps caused by political developments, and therefore he urged that upon termination of the war - at least in the scientific world - reconciliation between Germany, and the Allies be reestablished. He was thus deeply shocked, and protested violently, when in 1919 the Interallied Association of Academies was founded with Germany excluded. When, in spite of his and a few others’ protests, the Royal Netherlands Academy of Sciences and Letters decided to join the International Research Council (from which Germany was again excluded), he resigned his long-standing membership in the academy.
Views
The principal unknowns which Kapteyn tried to solve were the function D(r), that is, the space density of stars as a function of the distance r from the sun; and the function or the distribution of the stars according to brightness per unit volume. In a series of investigations by Kapteyn and his collaborators, extending over several decades, these functions became defined in more and more detail. Thus, the function D(r), initially determined only for stars generally without regard to their spectral type or galactic latitude or longitude, could later be determined separately for different types of stars, and <p(M) could be distinguished more and more according to both distance from the galactic plane and spectral type.
Kapteyn’s approach to these problems was basically different from that of contemporaries such as Hugo von Seeliger and Karl Schwarzschild. The latter proposed certain analytical expressions for the aforementioned functions, as well as for the distribution of observed quantities, and then tried to solve for the parameters involved by means of integral equations. Kapteyn, on the other hand, preferred the purely numerical approach, allowing full freedom for the form of the solution.
Reference has been made to the use of the proper motion, p, as well as the apparent magnitude, m, as an indicator of distance. In the early stages of his work, after having explored the use of the trigonometric parallaxes, Kapteyn emphasized the use of p rather than that of m because of the large spread known to exist among the absolute magnitudes. As a prerequisite to the application of the method, an attempt was made to determine the distribution of the stars according to their peculiar velocities; that is, of the velocities with respect to the mean motion of the stars, the latter, in turn, being the reflex of the motion of the sun.
During the early stages of Kapteyn's investigations, the approximate position and apparent brightness were known for somewhat less than a million stars; proper motions were known with varying degrees of accuracy for several thousand, and trigonometric parallaxes for fewer than 100 stars. Kapteyn encouraged efforts of many observatories to procure more data on trigonometric parallaxes, radial velocities, spectra, and proper motions; wherever possible, he assisted in the measurements of plates by means of the facilities of his growing laboratory. A carefully planned undertaking appeared in order, however, particularly because from the fainter stars a selection had to be made.
Kapteyn emphasized on many occasions the great need for improvement of the fundamental system of declinations and proposed observational methods to eliminate systematic errors. He demonstrated certain relations between the various spectral types of the stars and their kinematic properties and pursued especially the properties of the earliest types (the “helium stars”), for which the small ratio between peculiar velocity and solar motion allowed the determination of accurate individual parallaxes. The accounts of this latter work, in which Kapteyn’s approach to the handling of such delicate quantities as small proper motions is quite remarkable, are given in two extensive papers.
Kapteyn had an almost inexhaustible capacity for scientific activity. In his attitude toward research he was extremely critical, with respect both to his own work and to that of others. He never sacrificed clarity of treatment or exposure of essential details for elegance of presentation; and, although a mathematician himself by his early training, he strongly disliked treatises in which emphasis lay more on the form of the mathematical expression than on proper evaluation of the basic observations. It was only through his thorough knowledge of their strengths and weaknesses that he was able to draw proper conclusions from what were sometimes limited data.
Membership
In 1888 Kapteyn became a member of the Royal Netherlands Academy of Arts and Sciences. He was also a foreign member of the Royal Society, foreign associate of the National Academy of Sciences, corresponding member of the Saint Petersburg Academy of Sciences, and honorary fellow of the Royal Society of Edinburgh.
Royal Netherlands Academy of Arts and Sciences
,
Netherlands
Royal Society
,
United Kingdom
National Academy of Sciences
,
United States
Saint Petersburg Academy of Sciences
,
Russia
Royal Society of Edinburgh
,
United Kingdom
Personality
Kapteyn possessed the ability to conduct several large-scale undertakings at once, and to handle with great care and ability the more detailed matters essential to the successful completion of any task.
He had a keen sense of humor and was a celebrated lecturer to audiences of all kinds. In his relation to friends and colleagues, Kapteyn was very sensitive to friendship and cordiality. Having suffered in early youth from a lack of warmth and protection in his family life - his parents being fully occupied with their boarding school and perhaps having aimed at equal treatment of all their “children” - he later responded all the more readily to human relations.
Connections
Kapteyn married Catharina Elisabeth Kalshoven in 1879, and the couple had two daughters and one son. In the town of Groningen, where he lived for more than forty years with his wife and family, he was well remembered more than thirty years after his death.
From his collaboration with many colleagues grew close ties of friendship, such as that with Gill (with whom a regular correspondence developed over three decades).