Dutch astronomer. In 1927, he calculated the mass and size of our Galaxy, the Milky Way, and the Sun's distance from its centre, from the observed movements of stars around the Galaxy's centre. In 1950 Oort proposed that comets exist in a vast swarm, now called the Oort cloud, at the edge of the solar system.
In 1944 Oort's student Hendrik van de Hulst (1918- ) calculated that hydrogen in space would emit radio waves at 21 cm/8.3 in wavelength, and in the 1950s Oort's team mapped the spiral structure of the Milky Way from the radio waves given out by interstellar hydrogen.
Oort was born in Franeker, Friesland, and studied at Groningen. He spent most of his career at Leiden, becoming professor 1935 and director of the observatory 1945.
Oort confirmed the calculations of astronomers Bertil Lindblad and Harlow Shapley and went on to show that the stars in the Milky Way were arranged like planets revolving round a sun, in that the stars nearer the centre of the Galaxy revolved faster round the centre than those farther out.