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From Citizendium, the Citizens' Compendium

Jacobus Cornelius Kapteijn (internationally known as Jacobus Kapteyn), astronomer (Barneveld January 19, 1851 - Amsterdam June 18, 1922). Son of Gerrit Jacobus Kapteijn, who was a boarding school proprietor, and Elisabeth Cornelia Koomans. Jacobus was the ninth of fifteen children, many of whom would take an important position in life. Jacobus Kapteyn married Catharina Elisabeth Kalshoven on July 17 1879. One son and two daughters were born in this marriage.

Kapteyn's contributions are mainly in the field of stellar astronomy, in particular the study of the spatial distributions and motions of the stars. When he started his research the "structure of the universe" was identical to the spatial distribution of the stars found by telescopes. The idea did not yet exist that all the observed stars form just one galaxy (the Milky Way) and that there are numerous other galaxies. During Kapteyn's lifetime this was discovered and Kapteyn contributed importantly to the development of these ideas. In 1904, Kapteyn observed that stars could be divided into two streams, moving in nearly opposite directions. Kapteyn's data were the first evidence of the rotation of the Milky Way, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.