Jacobus Kapteyn (nonfiction)

Jacobus Kapteyn. Painting by Jan Veth (1921).

Prof Jacobus Cornelius Kapteyn FRS FRSE LLD (19 January 1851 – 18 June 1922) was a Dutch astronomer. He carried out extensive studies of the Milky Way and was the discoverer of evidence for galactic rotation. Kapteyn was also among the first to suggest the existence of dark matter using stellar velocities as early as 1922.