Jan Oort

Jan Hendrik Oort was a colossal figure in twentieth-century astronomy whose structural insights mapped the Milky Way galaxy and the furthest frozen fringes of the solar system. A student of Jacobus Kapteyn at Groningen, Oort spent most of his distinguished career as professor and director of the observatory at Leiden University.

In 1927, Oort analyzed stellar motions to confirm that the Milky Way is not stationary, but rotates differentially like a giant pinwheel, calculating that the solar system orbits the galactic center at 220 kilometers per second approximately 30,000 light-years from the core. In 1932, studying the vertical motions of stars perpendicular to the galactic plane, he provided the first dynamical evidence for unseen missing mass (early dark matter). In 1950, to explain where long-period comets originate, he deduced the existence of a vast spherical reservoir of billions of icy cometary nuclei surrounding the solar system at distances up to 100,000 astronomical units—now called the Oort cloud.