Editing Jan Oort (section)

==Career==
At Yale, Oort was responsible for making observations with the Observatory's [[zenith telescope]]. "I worked on the problem of latitude variation", he later recalled, "which is quite far away from the subjects I had so far been studying." He later considered his experience at Yale useful as he became interested in "problems of fundamental astronomy that [he] felt was capitalized on later, and which certainly influenced [his] future lectures in Leiden." Personally, he "felt somewhat lonesome in Yale", but also said that "some of my very best friends were made in these years in New Haven."<ref name="OHT" />

=== 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 />

=== Nazi invasion of Netherlands ===
In 1940, [[Nazi Germany]] [[Battle of the Netherlands|invaded the Netherlands]]. Soon after, the [[Reichskommissariat Niederlande|occupying regime]] dismissed all Jewish professors from [[Leiden University]] and other universities. "Among the professors who were dismissed", Oort later recalled, "was a very famous … professor of law by the name of Meyers. On the day when he got the letter from the authorities that he could no longer teach his classes, the dean of the faculty of law went into his class … and delivered a speech in which he started by saying, 'I won't talk about his dismissal and I shall leave the people who did this, below us, but will concentrate on the greatness of the man dismissed by our aggressors.'"<ref name= OHT />

This speech (26 November 1940) made such an impression on all his students that on leaving the auditorium they defiantly sang the [[Wilhelmus|anthem of the Netherlands]] and went on strike. Oort was present for the lecture and was greatly impressed. This occasion formed the beginning of the active resistance in Holland. The speech by [[Rudolph Cleveringa]], the dean of the faculty of Law and former graduate student of professor Meijers, was widely circulated during the rest of the war by the resistance groups. Oort was in a little group of professors in Leiden who came together regularly and discussed the problems the university faced in view of the German occupation. Most of the members of this group were put in hostage camps soon after the speech by Cleveringa. Oort refused to collaborate with the occupiers, "and so we went down to live in the [[Rural area|country]] for the rest of the war." Resigning from the Royal Academy, from his professorial post at Leiden, and from his position at the Observatory, Oort took his family to Hulshorst, a quiet village in the province of [[Gelderland]], where they sat out the war. In Hulshorst, he began writing a book on stellar dynamics.<ref name= NYT /><ref name= OHT />

=== Oort's radio astronomy ===
[[File:JanOort.jpg|thumb|Oort by an image of the galaxy [[Messier 81]].]]
Before the war was over, he initiated, in collaboration with a [[Utrecht University]] student, [[Hendrik van de Hulst]], a project that eventually succeeded, in 1951, in detecting the 21-centimeter radio emission from interstellar hydrogen spectral line at radio frequencies. Oort and his colleagues also made the first investigation of the central region of the Galaxy, and discovered that "the 21-centimeter radio emission passed un-absorbed through the gas clouds that had hidden the center from optical observation. They found a huge concentration of mass there, later identified as mainly stars, and also discovered that much of the gas in the region was moving rapidly outward away from the center."<ref name= OX2 /> In June 1945, after the end of the war, Oort returned to Leiden, took over as director of the Observatory, and became Full Professor of Astronomy.<ref name= NYT /> During this immediate postwar period, he led the Dutch group that built radio telescopes at [[Radio Kootwijk]], Dwingeloo, and Westerbork and used the 21-centimeter line to map the Milky Way, including the large-scale spiral structure, the [[Galactic Center]], and gas cloud motions. Oort was helped in this project by the Dutch telecommunications company, PTT, which, he later explained, "had under their care all the radar equipment that was left behind by the Germans on the coast of Holland. This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture.... Our radio astronomy was really started with the aid of one of these instruments… it was in Kootwijk that the first map of the Galaxy was made."<ref name= OHT /> For a brief period, before the completion of the Jodrell Bank telescope, the Dwingeloo instrument was the largest of its kind on Earth.

It has been written that "Oort was probably the first astronomer to realize the importance" of radio astronomy.<ref name= NYT /> "In the days before radio telescopes," one source notes, "Oort was one of the few scientists to realise the potential significance of using radio waves to search the heavens. His theoretical research suggested that vast clouds of hydrogen lingered in the spiral arms of the Galaxy. These molecular clouds, he predicted, were the birthplaces of stars."<ref name= ESA /> These predictions were confirmed by measurements made at the new radio observatories at [[Dwingeloo Radio Observatory|Dwingeloo]] and [[Westerbork Synthesis Radio Telescope|Westerbork]]. Oort later said that "it was [[Grote Reber]]'s work which first impressed me and convinced me of the unique importance of radio observations for surveying the galaxy."<ref name= OHT /> Just before the war, Reber had published a study of galactic radio emissions. Oort later commented, "The work of Grote Reber made it quite clear [radio astronomy] would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption."<ref name= OHT /> Oort's work in radio astronomy is credited by colleagues with putting the Netherlands in the forefront of postwar astronomy.<ref name= NYT /> Oort also investigated the source of the light from the Crab Nebula, finding that it was [[Polarization (waves)|polarized]], and probably produced by [[synchrotron radiation]], confirming a hypothesis by [[Iosif Shklovsky]].<ref name= oort-crab>{{cite book |title=A short biography of Jan Hendrik Oort: 7. Crab Nebula |publisher=Leiden University Library |first1=J. |last1=Katgert-Merkelijn |first2=J. |last2=Damen |chapter=Jan Oort, astronomer : Catalogue of an exhibition in Leiden University library, April 20 - May 27, 2000 |date=2000 |df=dmy-all |hdl = 1887/77628}}</ref>

=== Comet studies ===
Oort went on to study comets, which he formulated a number of revolutionary hypotheses. He hypothesized that the Solar System is surrounded by a massive cloud consisting of billions of comets, many of them "long-period" comets that originate in a cloud far beyond the orbits of [[Neptune]] and [[Pluto]]. This cloud is now known as the Oort Cloud. He also realized that these external comets, from beyond Pluto, can "become trapped into tighter orbits by [[Jupiter]], and become periodic comets, like [[Halley's comet]]." According to one source, "Oort was one of the few people to have seen Comet Halley on two separate apparitions. At the age of 10, he was with his father on the shore at Noordwijk, Netherlands, when he first saw the comet. In 1986, 76 years later, he went up in a plane and was able to see the famous comet once more."<ref name= OX2 />

In 1951 Oort and his wife spent several months in [[Princeton University|Princeton]] and [[Pasadena College|Pasadena]], an interlude that led to a paper by Oort and [[Lyman Spitzer]] on the acceleration of interstellar clouds by [[O-type stars]]. He went on to study high-velocity clouds. Oort served as director of the Leiden Observatory until 1970. After his retirement, he wrote comprehensive articles on the galactic center and on superclusters and published several papers on the quasar absorption lines, supporting Yakov Zel'dovich's pancake model of the universe. He also continued researching the Milky Way and other galaxies and their distribution until shortly before his death at 92.<ref name= NYT />

One of Oort's strengths, according to one source, was his ability to "translate abstruse mathematical papers into physical terms," as exemplified by his translation of the difficult mathematical terms of Lindblad's theory of differential galactic rotation into a physical model. Similarly, he "derived the existence of the comet cloud on the outskirts of the Solar System from the observations, using the mathematics needed in dynamics, but then deduced the origin of this cloud using general physical arguments and a minimum of mathematics."<ref name= NYT /><ref name= OX2 />