Who discovered that the Milky Way rotates?
This is credited to B. Lindblad in 1926 of Stockholm Sweden. In 1905, Kapteyn had discovered that the proper motions of nearby stars were not random when the solar motion was subtracted from them. Instead they moved along a stream headed towards Lyra. Lindblad showed that this was the result of the rotation of the Milky Way galaxy. Astronomer V. M. Slipher had also demonstrated a few years before Lindblad that other ‘external’ galaxies also rotated by looking at their spectra which were Doppler shifted in a way that indicates rotation. later, Jan Oort showed that the radial velocities of stars could be easily explained by a ‘differentially rotating’ galaxy and a very simple geometric calculation.
