Jan Hendrik Oort, without a doubt one of the greatest astronomers of the twentieth century, was born in the Dutch town of Franeker on April 28, 1900. At the age of 17 he went to Groningen University, taking his degree in astronomy in 1921. After a year at Groningen as an assistant, Oort spent two years at Yale Observatory. While in New Haven he was offered a position at Leiden Observatory, where he returned, in 1924, to spend the rest of his working life, apart from brief periods on sabbatical and a few years during World War II.
Oort is probably best known for showing that the motions of stars in the solar neighborhood reveal the effects of differential rotation. After publishing his results, he spent considerable time and effort generalizing them, building up a picture of the structure of the galaxy as a whole, and in the process founding the mathematical theory of galactic structure.
But differential galactic rotation was not Oort's only claim to fame. He is as well known for showing that the solar system is surrounded by a vast cloud of comets (now often referred to as an “Oort cloud”). By studying the orbits of long-period comets, Oort noticed that many of them seemed to originate in a zone far beyond the orbit of Pluto. Disturbances by passing stars can then perturb these external comets out of their roughly circular orbits, leading some of them into orbits that take them close to the sun, at which point they might become trapped into tighter orbits by Jupiter, and become periodic comets, like Halley's comet.
Another major contribution was Oort’s demonstration that the light from the Crab Nebula is strongly polarized. Oort also played an important role in the establishment of ESO, the European Southern Observatory.
In addition to the Vetlesen Prize, which he received in 1966, Dr. Oort was the recipient of many awards and honors for his pioneering work in astronomy. These included the Gold Medal of the Royal Astronomical Society, conferred in 1946, the Jansky Prize (1967), the Balzan Prize (1984), and the Kyoto Prize (1987), as well as eleven honorary degrees. He also served as general secretary of the International Astronomical Union (or IAU) from 1935 to 1948, and as its president from 1958 to 1961.
Jan Hendrik Oort died on November 5, 1992, leaving behind an impressive legacy in nearly every field of stellar astronomy and astrophysics.