1900 - Born on the 28th of April in Franeker, Friesland, Netherlands.
1921 - Studied in Groningen with Jacobus Cornelius Kapteyn. His Ph.D thesis was titled The stars of high velocity.
1924 - He discovered the Galactic Halo a group of stars orbiting the Milky Way.
1927 - He confirmed Bertil Lindblad's theory that the Milky Way galaxy rotates, by analyzing the movements of stars.
- He calculated that the center of the Milky Way was 5,900 parsecs (19,200 light years) from the Earth in the direction of the constellation Sagittarius.
1935 - He became professor at the Observatory of the University of Leiden.
1946 - Received a Gold Medal of the Royal Astronomical Society.
1950 - He raised funds for a new radio telescope in Dwingeloo.
- He was able to demonstrate that the light from the Crab Nebula was Polarized.
1951 - Awarded by Henry Norris Russell Lectureship.
1972 - Awarded by Karl-Schwarzschild-Medaille of the Astronomische Gesellschaft.
1992 - He died on the 5th of November in Leiden, Netherlands.