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.

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.

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.

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.