Discovering the rotation of our own galaxy

Abstract

In the early twentieth century, the Dutch astronomer, Jan Oort, made brilliant use of the two most basic measurements of the motions of nearby stars. These are motions away or towards the Sun based on the Doppler effect, and motion perpendicular to the line of sight, measured as a slow change in the position of the star on the sky. He was able to draw far-reaching conclusions about the rotation and the mass of the Milky Way galaxy in which our Sun is located. His arguments and calculations were based on a simple dynamical model. This model is introduced and described in this article. It can serve as an excellent exercise in an undergraduate physics course, illustrating both mechanical principles and basic astronomy.