Jan Hendrik Oort. Portrait of the Dutch astronomer Jan Hendrik Oort (1900-1992). Oort did most of his work in Leiden, Netherlands, and was director of the observatory there from 1945-70. He showed that the stars nearest the centre of our Galaxy have higher angular velocities than those further out. This proved that our Galaxy was rotating. In 1951, Oort was involved in the discovery of characteristic radio emissions from interstellar hydrogen clouds. He also proposed the existence of an area of cometary material (Oort cloud) at the edge of the solar system. These lumps of ice and dust sometimes move into orbit around the Sun to form comets.