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.