Dutch astronomer who analysed the structure of the universe by studying the distribution of stars using photographic techniques. To achieve more accurate star counts he introduced the technique of statistical astronomy.

Kapteyn was born in Barneveld and studied at Utrecht. After working at the observatory at Leiden, he was professor at Groningen 1875-1921.
Kapteyn entered into an arrangement with Cape Town Observatory in South Africa whereby photographs of the stars in the southern hemisphere were analysed at Groningen and published as Cape Photographic Durchmusterung 1896-1900. This presented data on the brightness and positions of nearly 455,000 stars.
Studying the proper motions of stars, Kapteyn reported 1904 that these were not random, as had been believed; 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.
In 1906 he selected 206 specific stellar zones, aiming to ascertain the magnitudes of all the stars within these zones, as well as to collect data on their spectral type, radial velocity, proper motion, and so on. This enormous project was the first coordinated statistical analysis in astronomy and involved the cooperation of over 40 different observatories.