Jan Hendrik Oort

Jan Hendrik Oort ( or ; 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. The New York Times called him "one of the century's foremost explorers of the universe"; the European Space Agency website describes him as "one of the greatest astronomers of the 20th century" and states that he "revolutionised astronomy through his ground-breaking discoveries." In 1955, Oort's name appeared in Life magazine's list of the 100 most famous living people. He has been described as "putting the Netherlands in the forefront of postwar astronomy".

Oort determined that the Milky Way rotates and overturned the idea that the Sun was at its center. He also analyzed the vertical motions of stars near the Sun, using this data to estimate the local gravitational field. He calculated how much mass must be present to account for the stellar motions perpendicular to the Galactic plane. Comparing this mass to the visible stars and gas, in 1932, he wrote, "This leads to the conclusion that there must be considerable amounts of invisible matter." The following year, Fritz Zwicky called this invisible matter “dunkle Materie”, German for dark matter. It is now believed that dark matter makes up approximately 84.5% of the total mass in the Universe, and its gravitational pull is responsible for "the clustering of stars into galaxies and galaxies into connecting strings of galaxies".

However, Oort's data have been challenged and his discovery may have been spurious.

Oort discovered the galactic halo, a group of stars orbiting the Milky Way but outside the main disk. Additionally Oort is responsible for a number of important insights about comets, including the realization that their orbits "implied there was a lot more solar system than the region occupied by the planets."

The Oort cloud, the Oort constants, Oort Limit, an impact crater on Pluto (Oort), and the asteroid 1691 Oort were all named after him.