After completion of his studies, Dr Nassau served for 2 years as instructor in mathematics at the University of Syracuse. In 1921 he commenced a life-long association with the Case School of Applied Science (later renamed Case Institute of Technology) in Cleveland, Ohio, by accepting the position of assistant professor of mathematics and astronomy. In 1924 Dr Nassau was named Director of the school's Warner and Swasey Observatory which was then only somewhat better equipped than a typical small-college astronomical facility. Under Dr Nassau's 35-year long stewardship, the Warner and Swasey Observatory became one of the world's leading astronomical research institutions in the field of galactic structure.
Dr Nassau returned to Great Britain in 1927 to spend a year at the University of Cambridge working under the guidance of Sir Arthur Eddington. Dr Nassau's first important astronomical publications were written during this stay in Cambridge.
At the Case School of Applied Science, Dr Nassau distinguished himself as an inspired teacher who excited a remarkable degree of interest in astronomical research among his students. His teaching abilities, however, were not limited to the classroom. In Cleveland, he was the city's leading popularizer of astronomy, and the founder of the Cleveland Astronomical Society. This society has brought many outstanding scientists to Cleveland as lecturers and has established an annual fellowship for young foreign astronomers to do research at Case Institute. It has also popularized astronomy in the city and supported the Warner and Swasey Observatory in its growth. In 1939, inspired by Dr Nassau's efforts in research and public education, the surviving relatives and associates of Messrs Warner and Swasey devoted a sum of money for expanding the observatory, and for acquiring a large Schmidt type telescope. With these funds, the celebrated Burrell telescope with 24 in aperture and 36 in spherical mirror was acquired.
Completion and testing of the telescope was delayed by World War II. During the hostilities, Dr Nassau administered Case Institute's physics department in addition to the observatory. Later he served as a commander in the U.S. Coast Guard. After the war, Dr Nassau worked on an impressive series of galactic structure studies with photographic material obtained with the Burrell telescope, the first large Schmidt-type telescope to be built. Astronomy is indebted to Dr Nassau for his pioneering efforts in promoting its construction and in demonstrating the rich potentiality of this type of instrument. When the Schmidt telescope was moved to a new location outside of Cleveland in 1957, the new observational facility housing the telescope was properly named the 'Nassau Astronomical Station'. After the Burrell telescope was removed from its former location, its place was taken by a new 36 in Cassegrain telescope constructed with funds that Dr Nassau was able to raise in the community.
Throughout the post-war period, when Dr Nassau was intensely active in galactic structure research, he managed to keep up his beloved task of teaching elementary astronomy. In addition, he undertook administrative duties of national importance, serving as treasurer of the American Astronomical Society for 8 years, and as chairman of the U.S. National Committee of the International Astronomical Union in the politically sensitive years from 1948 to 1956. In 1959 Dr Nassau retired from his official duties at the Case Institute of Technology, but not from astronomy. He remained an avid researcher until illness slowed his pace in the last year of his life. Dr Nassau died on 1965 May 11. He is survived by his wife and two sons.
A testimony to Dr Nassau's engaging personality is the fact that the vast majority of the nearly 100 scientific papers he published were written in collaboration with other investigators. In addition to astronomy, Dr Nassau had a highly developed appreciation for the humanities in general and for history, philosophy, and theology in particular. He was an articulate and witty conversationalist. Many scientific and honorary societies chimed Dr Nassau as a member, but perhaps the best index of the importance of his scientific work is the proliferation of large-sized Schmidt telescopes that followed his pioneering adoption of this instrument for astronomical research.