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Jacobus Kapteyn

Jacobus Cornelius Kapteyn (January 19, 1851 - June 18, 1922), Dutch astronomer.

Kapteyn was born in Barneveld, Netherlands. In 1868 he went to the University of Utrecht[?] to study mathematics and physics. In 1875, after having finished his thesis, he work for three years at the Leiden Observatory[?]. In 1878 he became the first Professor of Astronomy and Theoretical Mechanics at the University of Groningen and this until his retirement in 1921.

Between 1896 and 1900, lacking an observatory, he volunteered to measure photographic plates taken by David Gill[?], who was conducting a photographic suvey of the southern hemispehere stars at the Cape Town Observatory[?]. The results of this collaboration was the publication of Cape Photographic Durchmusterung, a catalog listing positions and magnitudes for 454,875 stars on the Southern Hemisphere.

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 our 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 40 different observatories.

Kapteyn retired in 1921 at the age of 70, 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-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's 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, and 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, at Amsterdam in 1922, that Robert Trumpler[?] determined that the amount of extinction 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 its center.

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