Oort Proves the Spiral Structure of the Milky Way.

In 1927, Jan Hendrik Oort published data that provided dynamical proof of the proper motion of the stars near Earth. This evidence established that the observed effects were the result of differential velocities that could best be explained as the result of movement in the spiral arm of a large galaxy. He found the galactic center in the direction of Sagittarius in opposition to the direction Jacobus Cornelius Kapteyn had found, but in accord with the findings of Harlow Shapley. Oort came to this study quite naturally, as he had been a student of Kapteyn, the famous Dutch astronomer who was active in the study of the structure of the Milky Way. At the age of seventeen, Oort went to the University of Gröningen to study with Kapteyn. In his elementary lectures, Kapteyn emphasized deriving interpretation from observation rather than hypotheses or conjectures; in many ways, Oort’s career followed this dictum.