Astronomy at Princeton was originally taught as a part of ``natural philosophy,'' usually by professors who were primarily mathematicians. Walter Minto, called from Scotland by President Witherspoon in 1787 as professor of mathematics and natural philosophy, was the author of a treatise on the newly discovered planet Uranus. Andrew Hunter, Jr., Class of 1772, held the title Professor of Mathematics and Astronomy, 1804-1808.
The College was proud of its Rittenhouse Orrery -- a clock-like planetarium -- acquired in 1771, but it had difficulty raising funds for an observatory.
After the Nassau Hall fire of 1802, the trustees asked John Maclean Sr., who then held the professorship of mathematics and natural philosophy, ``to select from the materials remaining after the rebuilding of the college such parts as may be necessary for the building of an Observatory,'' but nothing came of this proposal.
Astronomy began its development as a separate discipline under Stephen Alexander who, beginning as a tutor in mathematics in 1833, was appointed professor of astronomy in 1840. He gave the first separate course in astronomy, made observations on a small telescope in his home, published some twenty scientific papers, and by his patience and enthusiasm for his subject secured benefactions for tide Halsted Observatory, completed from his plans in 1872. This observatory was one of the inducements that lured Charles A. Young from Dartmouth as Alexander's successor in 1877; others were the promise of an observatory office and residence built on Prospect Avenue in 1878, and of a 23-inch telescope installed in the Halsted Observatory in 1882.
Young was the founder of a distinguished dynasty of observatory directors, who with the exception of a few years, have represented a direct line of teachers and students. Seven years following Young's retirement in 1905, Henry Norris Russell, who graduated from Princeton in 1897 and took his Ph.D. under Young in 1900, became director. On Russell's retirement thirty-five years later, he was succeeded by Lyman Spitzer, Jr., who graduated from Yale in 1935 and earned his doctorate at Princeton under Russell in 1938.
Young pioneered in solar spectroscopy, using a spectroscope of his own devising to study the colors given off by light from the sun in order to determine the elements present in the sun's atmosphere. He was the author of an authoritative work on The Sun and four textbooks, said by Harlow Shapley to be ``models of clearness and balanced presentation,'' on which most of the next generation of American astronomers were nurtured.
While Young was primarily an observer, his successor, Henry Norris Russell, was chiefly a theorist who used mathematics and physics to explain the observations of others. With Eddington and Milne in England and Karl Schwarzschild in Germany, Russell laid much of the foundation of modern astrophysics.
Carrying on Young's study of the chemical composition of the sun, Russell brought to the investigation exact physical theory that made it possible for the first time to specify the relative amounts of the different elements present in the sun's atmosphere. ``His detailed recipe for the solar atmosphere'' (in Professor Spitzer's words) is known to astronomers as the ``Russell mixture'' and has been widely used in theoretical work on the sun and the stars. The surprisingly overwhelming abundance of hydrogen which he found (92 percent by volume) was later confirmed by others for stars in general and exerted a profound influence on all of astronomy.
Russell also became a leading authority on stellar evolution -- the birth, growth, decay, and death of a star. He and the Danish astronomer, Einar Hertzsprung, independently discovered a certain regularity in the relationship between the brightness of stars, their colors, and their spectral class; the diagram illustrating this relationship is usually called the Hertzsprung-Russell diagram.
Under Russell, Princeton became the foremost center for the analysis of eclipsing variables: double stars -- so close together they appear as a single star -- whose orbital motion around one another, accompanied by periodic eclipses, results in a systematic change in brightness, which when measured can be used to learn the relative sizes of the two stars. Russell worked out the theory and method of analysis in 1912, and for many years the most precise measurements of light variations were made by his colleague, Raymond S. Dugan, in thousands of painstaking observations. Another colleague, Newton L. Pierce, who studied under Dugan, carried on his teacher's work, using electronic techniques.
Professors Russell, Dugan, and John Q. Stewart collaborated on a two-volume textbook (1927) that ``essentially revolutionized astronomical teaching and astronomical textbook writing in America.''
On Russell's retirement in 1947 Lyman Spitzer, Jr., succeeded him as director of the Observatory and, several years later, as Charles A. Young Professor of Astronomy. At the same time Martin Schwarzschild, then thirty-five, was called from Columbia as professor of astronomy; he was later appointed Eugene Higgins Professor of Astronomy. The appointment of Spitzer, one of Russell's students, and of Schwarzschild, son of Russell's famous contemporary, Karl Schwarzschild, led to a dramatic expansion in Princeton's role as a center of astronomical research, mostly in the two fields of theoretical astrophysics and of observational space astronomy. This expansion has been financed in part by University funds, in part by substantial grants from the federal government through the Office of Naval Research, the National Science Foundation, the National Aeronautics and Space Administration, and the Air Force.
Spitzer has continued Russell's work in theoretical astrophysics, making pioneering calculations of the temperature of the very rare gases in interstellar space and investigating many aspects of the problem of star formation. Schwarzschild has also carried on the study of stellar evolution begun by Russell, with his investigations on the interior of stars, confirming, for the interior of the sun, Russell's discovery of the great abundance of hydrogen and helium in the solar atmosphere. By careful calculation of the changes in the interior of a star due to the burning of its nuclear fuel, Schwarzschild has been able to find its age corresponding to a given present position on the Hertzsprung-Russell diagram.
In 1951 Spitzer instituted on the Forrestal Research Campus a program (originally called Project Matterhorn but later named the Plasma Physics Laboratory) for studies of basic plasma physics and the possibilities of controlled thermonuclear power. This program brought an influx of specialists in plasma physics who have given Princeton great strength in a basic subject that may hold the key for solving the future energy problems of mankind: John M. Dawson, Edward A. Frieman, Harold P. Furth, Melvin B. Gottlieb, Martin D. Kruskal, Russell Kulsrud, Carl R. Oberman, Thomas H. Stix, and Shoichi Yoshikawa.
Schwarzschild has pioneered in the use of giant balloons to hoist telescopes 80,000 feet into the stratosphere, to get clearer pictures of the sun and other stars and planets. In a series of flights in 1957 and 1959 a 12-inch balloon telescope (Stratoscope I) enabled Schwarzschild and his associates to obtain the sharpest and most detailed photographs of the sun ever taken. A 36-inch balloon telescope (Stratoscope II) was developed in 1963 and was used to obtain unprecedentedly sharp photographs of planets and stellar systems. Before his death in 1976, Professor Robert E. Danielson was closely associated with Schwarzschild in this work.
Spitzer has also been responsible for conceiving and developing, as part of NASA's Orbiting Astronomical Observatory program, a special satellite project to study the composition and physical structure of inter-stellar gases and dust clouds in space. This equipment, which was launched in August 1972 on a satellite named Copernicus, contains a 32-inch telescope, an ultraviolet spectrometer, and auxiliary electronic equipment to measure the ultraviolet light absorption characteristics of cosmic clouds and the gaseous atmospheres of the brighter stars. Copernicus is in earth orbit at a height of about 500 miles, well beyond the 30- or 40-mile ozone layer, where the strong absorption of ultraviolet light by the ozone precludes such measurements. Astronomers from all over the world have come to Princeton to use this instrument in their researches. John B. Rogerson has been associated with Spitzer on this project; other members of the group have been Kurt Dressler, Donald C. Morton, Donald G. York, and Edward B. Jenkins.
Morton has headed up another innovative program in space astronomy. By means of equipment carried 80 miles high in a rocket, he and his associates were able to make observations of stars from above the earth's atmosphere. In 1965 they launched a rocket carrying a telescope with photographic equipment, which obtained for the first time detailed spectra of two stars in ultraviolet wavelengths which do not reach Earth's surface. Since that time other Princeton rocket-carried telescopes have obtained additional such information previously denied to astronomers. In particular, this program has unexpectedly shown that hot stars are blowing their atmospheres out into space at speeds of thousands of miles per second.
The department's facilities were greatly enhanced in 1966 by the installation of a 36-inch reflecting telescope in the FitzRandolph Observatory, which had been built in 1934 to replace the Halsted Observatory, and by the completion of the department's modern home, Peyton Hall, which replaced its former headquarters in the old Observatory of Instruction on Prospect Avenue.
The appointment in 1971 of John N. Bahcall as professor of astronomy in the Institute for Advanced Study, succeeding Bengt Str”mgren, who first held this position, further intensified the light given off by Princeton as a center for the astrophysical sciences, as has the presence from time to time, as visiting professors at the Princeton Observatory, of luminaries such as Walter Baade (Mt. Wilson Observatory), Subrahmanyan Chandrasekhar (Yerkes Observatory), Fred Hoyle (England), and Jan H. Oort (The Netherlands).
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