Duncan, John Charles
Born Knightstown, Indiana, USA, 8 February 1882
Died Chula Vista, California, USA, 10 September 1967
John Duncan discovered the expansion of the filaments of the Crab Nebula and several variable stars in the “spiral nebula” M33.
Duncan, son of Daniel and Naomi (néeJessup) Duncan, studied at Indiana University, earning his A.B. (1905) and M.A. (1906) degrees. He then proceeded to the University of California at Berkeley, where he studied under Lick Observatory director William Campbell and obtained his Ph.D. in 1909. Duncan married Katharine Armington Bullard in 1906. The Duncans had one daughter, Eunice Naomi (Strickler).
While still an undergraduate, Duncan taught at a school in rural Indiana from 1901 to 1903. In 1905/1906, Percival Lowell , impressed by the work of Indiana-trained students, established a fellowship for Indiana graduates, and Duncan was named the first Lawrence Fellow at the Lowell Observatory. There, he took part in the first photographic search for a trans-Neptunian planet. Duncan later returned to Lowell to aid with the search in the summer of 1912.
Following the receipt of his doctorate, Duncan was appointed as an instructor at Harvard University from 1909 until 1916 and concurrently at Radcliffe College (1911–1916). From 1916 to 1950, he served as chairman of the astronomy department at Wellesley College and director of its Whitin Observatory. Following his retirement from Wellesley, Duncan became a visiting professor at the University of Arizona’s Steward Observatory (1950–1962).
In the tradition of Lick Observatory students of his era, Duncan’s dissertation involved a spectrographic study of two Cepheid variable stars, Y Sagittarii and RT Aurigae. To try and explain the asymmetries present in the light curves of these stars, Duncan put forth a new hypothesis that linked the observed changes to supposed interactions between a pair of stars closely orbiting one another. Today, however, Cepheid variables are known to be single, pulsating stars.
Much of Duncan’s later work was conducted at Mount Wilson Observatory, which he first visited in 1920/1921, and thereafter as a voluntary researcher during his summers from 1922 to 1949. Duncan became a talented photographer, and many of his photographs of nebulae and galaxies were reproduced in astronomy textbooks from the 1920s through the 1950s. He wrote an introductory college textbook, Astronomy(first edition, 1926), which passed through several editions. Duncan also published an abbreviated textbook, Essentials of Astronomy(first edition, 1942), and coauthored a laboratory manual. He developed a number of teaching aids in the discipline.
By comparing photographic plates exposed at different epochs, Duncan discovered a rapid expansion of the filaments in the Crab Nebula, reminiscent of the growth of an envelope around Nova Persei in 1901. Duncan returned to this problem in 1938 and demonstrated that further expansion had taken place. We now understand these filaments to be the remains of a supernova, or exploding star, which was witnessed by Asian astronomers in the year 1054.
Duncan also examined plates he had taken of the “spiral nebula” M33. While searching for novae, he discovered three faint variable stars in 1920. By mid-1922, Duncan had followed their variations on 17 plates but he neither determined whether they had fixed periods nor suggested that they might be Cepheids. His discovery of variable stars in “spiral nebulae,” along with independent discoveries by Max Wolf and Walter Baade in Germany, prepared the way for Edwin Hubble ’s discovery of a Cepheid variable star in the “spiral nebula” M31. Hubble thereupon established that the “spiral nebulae” were in fact distant galaxies that lay well beyond the confines of the Milky Way.
Duncan’s long-exposure photographs, taken with the world’s largest telescopes, contributed to scientific knowledge and the popularization of science. His textbook remained one of the best introductions to the subject for 30 years. Duncan’s observations of the Crab Nebula and of M33 placed him on the cusp of major developments in our knowledge of supernovae and the study of external galaxies.
Duncan was elected a fellow of the Royal Astronomical Society and secretary of the American Astronomical Society (1936–1939). He was a member of the International Astronomical Union, American Association for the Advancement of Science, and numerous other professional organizations. His papers and letters may be found in the archives of Wellesley College, and at the observatories where he worked: Harvard, Lowell, Arizona, Lick, and Mount Wilson.
- Anon. (1973). “Duncan, John Charles.” In National Cyclopedia of American Biography. Vol. 54, pp. 88–89. Clifton, New Jersey: James T. White and Co.Google Scholar
- Dieke, Sally H. (1971). “Duncan, John Charles.” In Dictionary of Scientific Biography, edited by Charles Coulston Gillispie. Vol. 4, p. 249. New York: Charles Scribner’s Sons.Google Scholar