Editing Jan Oort (section)

=== Early discoveries ===
In 1924, Oort returned to the Netherlands to work at [[Leiden University]], where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930, and Professor Extraordinary in 1935.<ref name= NYT /> In 1926, he received his doctorate from Groningen with a thesis on the properties of high-velocity stars. The next year, Swedish astronomer [[Bertil Lindblad]] proposed that the rate of rotation of stars in the outer part of the galaxy decreased with distance from the galactic core, and Oort, who later said that he believed it was his colleague [[Willem de Sitter]] who had first drawn his attention to Lindblad's work, realized that Lindblad was correct and that the truth of his proposition could be demonstrated observationally. Oort provided two formulae that described galactic rotation; the two constants that figured in these formulae are now known as "Oort's constants".<ref name= NYT /> Oort "argued that just as the outer planets appear to us to be overtaken and passed by the less distant ones in the solar system, so too with the stars if the Galaxy really rotated", according to the ''Oxford Dictionary of Scientists''.<ref name=OX>{{cite book|title=Oxford Dictionary of Scientists|date=1999|publisher=[[Oxford University Press]]|location=Oxford|isbn=0-19-280086-8|page=211|url=https://books.google.com/books?id=AtngooiwXikC|access-date=2 June 2014}}</ref> He "was finally able to calculate, on the basis of the various stellar motions, that the Sun was some 30,000 light-years from the center of the Galaxy and took about 225 million years to complete its orbit. He also showed that stars lying in the outer regions of the galactic disk rotated more slowly than those nearer the center. The Galaxy does not therefore rotate as a uniform whole but exhibits what is known as 'differential rotation'."<ref name= OX2>{{cite book|title=Oxford Dictionary of Scientists|date=1999|publisher=Oxford University Press|location=Oxford|isbn=0-19-280086-8|page=411|url=https://books.google.com/books?id=AtngooiwXikC|access-date=2 June 2014}}</ref>

These early discoveries by Oort about the Milky Way overthrew the [[Kapteyn system]], named after his mentor, which had envisioned a galaxy that was symmetrical around the Sun. As Oort later noted, "Kapteyn and his co-workers had not realized that the absorption in the galactic plane was as bad as it turned out to be."<ref name= OHT /> Until Oort began his work, he later recalled, "the Leiden Observatory had been concentrating entirely on positional astronomy, meridian circle work and some proper motion work. But no astrophysics or anything that looked like that. No structure of the galaxy, no dynamics of the galaxy. There was no one else in Leiden who was interested in these problems in which I was principally interested, so the first years I worked more or less by myself in these projects. De Sitter was interested, but his main line of research was celestial mechanics; at that time the expanding universe had moved away from his direct interest."<ref name= OHT /> As the European Space Agency states, Oort "sh[ook] the scientific world by demonstrating that the Milky Way rotates like a giant '[[Breaking wheel|Catherine Wheel]]'." He showed that all the stars in the galaxy were "travelling independently through space, with those nearer the center rotating much faster than those further away."<ref name= ESA />

This breakthrough made Oort famous in the world of astronomy. In the early 1930s he received job offers from [[Harvard University|Harvard]] and [[Columbia University]], but chose to stay at Leiden, although he did spend half of 1932 at the [[Perkins Observatory]], in [[Delaware, Ohio|Delaware]], [[Ohio]].<ref name= NYT  />

In 1934, Oort became assistant to the director of [[Leiden Observatory]]; the next year he became General Secretary of the [[International Astronomical Union]] (IAU), a post he held until 1948; in 1937 he was elected to the [[Royal Academy]]. In 1939, he spent half a year in the U.S., and became interested in the [[Crab Nebula]], concluding in a paper, written with American astronomer [[Nicholas Mayall]], that it was the result of a [[supernova]] explosion.<ref name= NYT />