Kapteyn was born in
Barneveld to Gerrit J. and Elisabeth C. (née Koomans) Kapteyn, and went to the
University of Utrecht to study
mathematics and
physics in 1868. In 1875, after having finished his
thesis, he worked for three years at the
Leiden Observatory, before becoming the first
Professor of Astronomy and Theoretical
Mechanics at the
University of Groningen, where he remained until his
retirement in 1921. In 1888 he became a member of the
Royal Netherlands Academy of Arts and Sciences. Between 1896 and 1900, lacking an observatory, he volunteered to measure
photographic plates taken by
David Gill, who was conducting a photographic survey of
Southern Hemisphere stars at the
Royal Observatory at the Cape of Good Hope. The results of this collaboration was the publication of
Cape Photographic Durchmusterung, a catalog listing positions and magnitudes for 454,875
stars in the
Southern Hemisphere. In 1897, as part of the above work, he discovered
Kapteyn's Star. It had the highest
proper motion of any star known until the discovery of
Barnard's Star in 1916. In 1904, studying the
proper motions of stars, Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of the Milky Way Galaxy, which ultimately led to the finding of
galactic rotation by
Bertil Lindblad and
Jan Oort. In 1906, Kapteyn launched a plan for a major study of the distribution of stars in the Galaxy, using counts of stars in different directions. The plan involved measuring the
apparent magnitude,
spectral type,
radial velocity, and
proper motion of stars in 206 zones. This enormous project was the first coordinated statistical analysis in astronomy and involved the cooperation of over forty different observatories. He was awarded the
James Craig Watson Medal in 1913. Kapteyn later retired in 1921 at the age of seventy, but on the request of his former student and director of
Leiden Observatory Willem de Sitter, Kapteyn went back to Leiden to assist in upgrading the observatory to contemporary astronomical standards. His life's work,
First attempt at a theory of the arrangement and motion of the sidereal system, was published in 1922, and described a lens-shaped
island universe of which the density decreased away from the center, now known as the
Kapteyn Universe. In his model the Galaxy was thought to be 40,000
light years in size, the
Sun being relatively close (2,000 light years) to its center. The model was valid at high
galactic latitudes but failed in the
galactic plane because of the lack of knowledge of
interstellar absorption. It was only after Kapteyn's death, in
Amsterdam, that
Robert Trumpler determined that the amount of
interstellar reddening was actually much greater than had been assumed. This discovery increased the estimate of the galaxy's size to 100,000 light years, with the Sun replaced to a distance of 30,000 light years from the
Galactic Center. The astronomy institute of the
University of Groningen is named after Kapteyn. A street in the city of Groningen is also named after Kapteyn: the J.C. Kapteynlaan. And the
Isaac Newton Group of Telescopes on
La Palma in the
Canary Islands named the
Jacobus Kapteyn Telescope (JKT) after him. His daughter Henrietta (1881-1956) married astronomer
Ejnar Hertzsprung. ==Honours==