Jacobus Cornelis (or Cornelius) Kapteyn, Dutch astronomer, was a son of Gerrit J. Kapteyn and his wife Elisabeth C. Koopmans. He studied mathematics and physics at the University of Utrecht from 1868 and was awarded a doctoral degree in 1875. After three years as an astronomer at Leiden Observatory he was appointed professor of astronomy and celestial mechanics at the University of Groningen in 1878. He remained there until his retirement in 1921. During these years his main interest was in stars on a mass basis, and much of his work was aimed at elucidating the shape of the Milky Way galaxy and the large scale movement of stars within it. He was the author of well over a hundred scientific publications.
Kaptey started corresponding with David Gill*, director of the Royal Observatory, Cape of Good Hope, in 1885 and the two became good friends. Towards the end of 1885 Kapteyn agreed to undertake the huge task of systematically measuring and cataloguing the stars on the photographic plates of Gill's Cape photographic durchmusterung, with a view to compiling the first comprehensive photographic star catologue of the southern hemisphere. The results of twelve years of painstaking work were published, under the names of Gill and Kapteyn, as the Cape photographic durchmusterung for the equinox 1875 in three volumes of the Annals of the Cape Observatory (1896, 1897, 1900). The catelogue lists the positions and magnitudes of 454 875 stars to magnitude 9, in the region 18 degrees south declination to the South Pole.
Kapteyn then gave attention to the statistical study of proper motions - the gradual change of position of stars on the star sphere. The motions were considered at the time to be distributed randomly, with the addition of a parallactic component reflecting the movement of the sun. He found, however, that many stars were moving over the sky in roughly the same direction and with approximately the same speed. By 1904 he concluded that most relatively nearby stars belong to one of two streams, moving in opposite directions in the plane of the Milky Way. The next year he attended the joint meeting of the British and South African Associations for the Advancement of Science in South Africa as an invited expert, serving as vice-president of Section A of the British Association (though he was not a member). In a paper on "Star streaming", published in the Report of the meeting (pp. 257-265), he explained his method of study and announced his discovery. During this visit he and a number of other delegates were awarded honorary Doctor of Science (DSc) degrees by the University of the Cape of Good Hope.
The following are some of Kapteyn's major publications: Components t and v of the proper motions... for the stars of Bradley (1900); The proper motion of the Hyades... (1904); The distribution of the stars in space (1909); The parallax of the Hyades... (1909); The proper motion of 3714 stars derived from plates taken at the observatories of Helsingfors and Cape of Good Hope (1914); and On the parallaxes and motion of the brighter galactic helium stars... (1918).
The mass study of proper motions enabled him to detect, in any group of stars, the size of the parallactic common motion imposed upon all of them by the motion of the solar system. This paralactic motion is smaller for distant groups of stars than for nearby ones, and its measurement enabled him to estimate stellar distances beyond the limits of the methods then in use. During this work he found, with the help of R.T.A. Innes* at the Cape, a star with the largest known proper motion, which came to be known as Kapteyn's star. (Barnard's star was later found to have an even larger proper motion).
From around 1906, with the cooperation of many observatories, Kapteyn organised star counts in randomly selected regions of the sky and used the resulting numbers to derive the approximate shape of the galaxy. Though his answers have not stood the test of time, the project marks the beginning of statistical astronomy.
Kapteyn became a member of the Royal Netherlands Academy of Sciences in 1888. He was elected a fellow of the Royal Society of London in 1919, and was a member also of the French Academy of Sciences. He was awarded, among others, the gold medal of the Royal Astronomical Society in 1902. An asteroid, a crater on the moon and the Kapteyn Astronomical Institute at the University of Groningen were named after him.