Background

Jan Hendrik Oort was born on April 28, 1900, in Franeker, Netherlands to the family of a physician Abraham Hermanus Oort and Ruth Hannah Faber. He was one of five children in the family. His father was a psychiatrist and the family left Franeker shortly after Oort's birth when his father became the director of a psychiatric clinic in Oegstgeest near Leiden.

Education

Oort went to primary school in Oegstgeest and then to Leiden for secondary school. Encouraged by his parents to follow his interests, Oort decided to study physics at the University of Groningen, which he entered in 1917. After attending lectures by the famous astronomer Jacobus Cornelius Kapteyn, Oort was inspired to switch to astronomy. He later recalled that the most important lessons that Kapteyn taught him as an undergraduate were to link interpretations directly to observations and to be very skeptical about speculations and hypotheses. He put these lessons into effect throughout his career. The high-velocity stars formed the subject of Oort’s doctoral thesis, which he defended at Groningen in May 1926.

Oort took his doctoral examination in 1921. This marked the completion of coursework for the degree of doctor of science, but attaining the higher degree would require several more years of study. After the examination Oort became the assistant to Kapteyn’s collaborator, Pieter Johannes Van Rhijn, at Groningen. But after a year Oort left to gain experience in the United States. From 1922 to 1924, Oort was an assistant to Frank Schlesinger at the Yale Observatory. Schlesinger was a great authority on fundamental positional astronomy. Oort was trained in astrometry, in particular, the use of a zenith telescope to secure very accurate positions of stars. He later judged the experience at Yale as useful, but it did not mesh well with his own aspirations. Hence when Willem de Sitter, director of the observatory at Leiden, offered him a position there, Oort quickly accepted.

Career

After studies at the University of Groningen, Oort was appointed astronomer to the Leiden Observatory in 1924. Within just a few years of completing his dissertation, Oort had propelled himself into the front rank of younger astronomers. Job offers followed. In 1930 Harvard invited him to become the Willson Professor of Astronomy and in 1932 Columbia University wanted him as the director of its astronomy department. He considered both offers seriously but in the end, he chose to stay at Leiden, where he remained for the rest of his career.

Leiden, however, did have its drawbacks. There were no large optical telescopes there (the biggest were a 26-cm visual refractor and a 33-cm astrometric telescope, and both dated from the nineteenth century), and anyway, Holland was a poor location from which to pursue optical astronomy. In 1923 an agreement was reached whereby Leiden astronomers had access to the Union Observatory in Johannesburg, South Africa. But the instruments available to Oort there, as at Leiden, were much less powerful than the best of the telescopes in the United States. Hence Oort made a half-year visit in 1932 to the Perkins Observatory in Delaware, Ohio. There he photographed galaxies with a 60-inch reflector in order to measure their luminosity distributions. His goals were to investigate the bulges in spiral galaxies and the forces operating within elliptical galaxies so as to better understand the development and maintenance of spiral structures. But he had very little experience of this sort of observing and overall he was not especially successful.

During the 1930s Oort continued to work principally at problems of galactic structure and dynamics. With Oort's growing seniority he assumed extra administrative tasks. In 1934 Willem de Sitter died and Ejnar Hertzsprung became director of the Leiden Observatory. Oort became Hertzsprung's deputy. Oort did not relinquish this position until 1948, although much of the administrative burden in the war years was carried by Walter Adams at the Mount Wilson Observatory in the United States because of the disruption of communications in war-torn Holland. Oort continued to play a leading part in the running of the union and he would serve as its president from 1958 to 1961.

Oort spent part of 1939 in the United States. He arrived back in the Netherlands days before the outbreak of World War II. With the German occupation of the country in 1940, conditions became at best difficult for astronomical research, and communications with astronomers outside the Netherlands slowed to a trickle by late 1941.

Oort did continue some research, however, including on the Crab Nebula, generally identified as the result of a stellar explosion. During his 1939 United States visit, Oort and Nicholas Ulrich Mayall at the Lick Observatory had discussed data on the Crab. These discussions led to papers in 1942, although the final manuscript of one listing Mayall and Oort as coauthors never reached Oort. Oort, however, had authorized Mayall to publish it if this happened.

In 1942 Jewish professors were dismissed from the University of Leiden. Oort was a member of a group of professors who met regularly to discuss the problems of the occupation and the Nazification of the university. After a number of these professors were put into detention camps, Oort, to avoid the same fate, decided to leave for the small village of Hulshorst some 100 kilometers from Leiden (later, in 1944, he moved to Nunspeet). He also resigned from his posts at the university. In effect, he disappeared from view, at least as far as the Germans were concerned.

Oort, nevertheless, stayed in touch with the observatory and was able to continue some astronomical research. On occasion, he cycled back to Leiden. He continued to organize work and to deliver unauthorized lectures. Oort also attended scientific meetings of the Nederlandse Astronomenclub.

Despite the isolation of Holland from the outside astronomical world, Oort had come across a paper by an American engineer, Grote Reber, in a copy of the Astrophysical Journal smuggled into the country. Here Reber reported on his investigations of radio emissions from the Milky Way.

The end of the war meant a rapid end to Hertzsprung's directorship of the Leiden Observatory. Oort succeeded him and also became a full professor. Much of Oort's time was now devoted to improving conditions at the observatory and attending to administrative tasks. But he was also itching to press forward with the radio researches he had started in the war.

Oort retired in 1970. With retirement, he was freed of administrative duties but he continued to be very active in research until well into his eighties, and the last piece he wrote for an astronomical journal was published in the year he died.

Achievements

• By the time of his death, at the age of 92, Jan Oort was recognized as one of the greatest astronomers of the 20th century. Both an observer and a theorist, Oort revolutionized astronomy through his ground-breaking discoveries. Oort received some of the highest awards in astronomy, including the 1966 Vetliesen Prize from Columbia University. Asteroid 1691 Oort (1956 RB), discovered September 9, 1956, by Ingrid van Houten-Groeneveld and Karl Wilhelm Reinmuth was named in his honor.

Works

• book

  • Origin and development of comets: The Halley lecture for 1951, delivered at Oxford on May 1 1951
  • Transactions of the International Astronomical Union - Volume 7 1950

Religion

Jan Oort's grandfathers were notable Protestant clergymen and he was baptized and raised in this faith.

Politics

Jan Oort wasn't involved in politics and collaborated wi every government that could support his research excep for the German one diring the Nazi occupation of the Netherlands.

Views

In 1925 Bertil Lindblad of Sweden had advanced the theory that the Milky Way rotates in its own plane around the center of the galaxy. Oort was able to confirm this theory in 1927 through his own direct observations of star velocities in the galaxy, and he modified the theory substantially into the form used thereafter.

Oort’s subsequent work, as well as that of the school of astronomy he developed in the Netherlands, was directed toward strengthening and testing the Lindblad-Oort theory. In the early 1950s Oort used radio astronomy to determine that the Sun is about 30,000 light-years from the center of the galaxy and takes 225 million years to complete an orbit around it. The discovery in 1951 of the 21-cm radio waves generated by hydrogen in interstellar space provided him with a new method for mapping the spiral structure of the galaxy.

In 1950 Oort proposed that comets with very long periods originate from a vast cloud of small bodies that orbit the Sun at a distance of about one light-year, and the approach of other stars toward this cloud alters some comets’ orbits so that they pass close to the Sun. The existence of this region, which was named the Oort Cloud, eventually came to be accepted by most astronomers.

Like his mentor Kapteyn, Oort was a convinced internationalist when it came to science, and in 1935 his standing in the wider international astronomical community was also underlined when he became general secretary to the International Astronomical Union, the leading international organization of astronomers.

Membership

From 1958 to 1961 Oort was president of the International Astronomical Union, of which he had been general secretary from 1935 to 1948.

• 

  Royal Society , United Kingdom

• 

  Academy of Sciences of the Soviet Union , Soviet Union

• 

  Royal Swedish Academy of Sciences , Sweden

• 

  Royal Netherlands Academy of Arts and Sciences , Netherlands

• 

  Pontifical Academy of Sciences , Holy See

• 

  American Academy of Arts and Sciences , United States

• 

  German Academy of Sciences Leopoldina , Germany

• 

  National Academy of Sciences , United States

• 

  From 1958 to 1961 Oort was the president of the International Astronomical Union, of which he had been the general secretary from 1935 to 1948.

  International Astronomical Union

Personality

Jan Oort was friendly and courteous in his personal dealings although some students found him intimidating as he continually urged them to probe more deeply into a problem.

Interests

• Writers

  Jules Verne

Connections

Jan Oort married Mieke Graadt van Roggen, whom he had first met at a university celebration. The couple had three children between 1928 and 1934, Coenraad, Abraham, and Marijke, and their happy marriage lasted until Oort’s death sixty-five years later.

Father:

Abraham Hermanus Oort

Mother:

Ruth Hannah Faber

Wife:

Mieke Graadt van Roggen

Son:

Coenraad Oort

Son:

Abraham Oort

Daughter:

Marijke Oort

teacher:

Jacobus Kapteyn

Acquaintance:

Edwin Hubble

References

• Jan Hendrik Oort: Master of the Galactic System
  2019
• The Letters and Papers of Jan Hendrik Oort: As Archived in the University Library, Leiden
  1997
• The Rise of Radio Astronomy in the Netherlands: The People and the Politics
  2017
• Oort and the Universe: A Sketch of Oort’s Research and Person
  1980