Oort, Jan Hendrik (1900–1992)

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 the University of Groningen, receiving his PhD in 1926. After two years (1922–24) as research assistant at Yale University Observatory, Connecticut, he went to the University of Leiden in 1924, where he spent most of his career, becoming professor of astronomy in 1935 and director of the observatory 1945–70.

Oort confirmed the calculations of Swedish astronomer Bertil Lindblad and US astronomer Harlow Shapley on the size and shape of our Galaxy, 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.

He established radio observatories at Dwingeloo and Westerbork, which put the Netherlands in the forefront of radio astronomy.

He played an important part in the national and international organization of astronomy, as a founder of the European Southern Observatory, La Silla, Chile, in 1962, and as secretary (1935–48) and president (1959–61) of the International Astronomical Union. He was the first recipient of the Vetlesen Prize, established in 1966. One of the major awards in the earth sciences, the Vetlesen Prize recognizes achievement in the sciences resulting in a clear understanding of the earth, its history, or its place in the universe.