Jan Oort received his higher education at the University of Groningen where he obtained a Ph.D. in 1926. He began working at the Leiden Observatory in 1924, and became its director in 1945.

In 1927, Oort had been able to demonstrate that the Milky Way galaxy was rotating. Since a galaxy is not a solid object, but comprised of billions of stars, it does not rotate as a single solid object. Instead, stars near the center of the galaxy move faster, while those farther move slower. By studying the motion of the stars in our vicinity, Oort was able to determine that the center of the galaxy lay in the same direction as the constellation of Sagittarius. That was in agreement with the observations of American astronomer Harlow Shapley. Shapley, however, had placed the galactic center at a distance of 50,000 light-years. Oort, taking into account the discovery of dark dust clouds...