Colors and Motions

Kapteyn had drawn the attention of Simon Newcomb of the US Naval Observatory, who visited him in Groningen and at his second home in Vries. Together with his first PhD student Willem de Sitter (who had worked at the Cape), he commenced the study of the distribution of the colors of stars in different regions of the sky. At that time Kapteyn started to look at patterns of stellar proper motions on the sky, which could be used to estimate distances of stars statistically. In Groningen he was very active in the Physical Sciences Society, which promoted the understanding of scientific progress among a wide audience.