# John Charles Duncan

John Charles Duncan (February 8, 1882 – September 10, 1967) was an American astronomer. His work spanned astronomy’s transition from a focus on observation and location measurement to astrophysics. He was well known for his basic college textbook "Astronomy", in widespread use for 30 years after its first publication in 1926. His career was a fruitful combination of research and teaching at major observatories, in his own classrooms and through his textbook. Duncan was the first to note the expansion of the Crab Nebula and from that determine the approximate year of its creation, discovered variable stars in what were soon found to be distant galaxies, and describe the nebular structures now known as the Pillars of Creation.

## Early life

John Charles Duncan was born at home at Duncan's Mill, near Knightstown, Ind., on Feb. 8, 1882. He was the third child of Daniel Davidson and Naomi Jessup Duncan.
Daniel was born in Ohio in 1844, the family settling in Henry County, Indiana in 1856. During the Civil War he served briefly as a Private in Company A, 105th Indiana Infantry, which was organized to repel Morgan's Raid. With his father Samuel he was a flour miller at S. & D.D. Duncan, 3 miles northeast of Knightstown on the Big Blue River. After Samuel died, the firm became D.D. Duncan, and later Duncan's Mill. Naomi was born in 1842 in Henry County, the daughter of a Quaker family.

## Education

Duncan graduated from Knightstown High School in 1899, at a time when the graduation rate of American 17 year olds was 6.4%.
Duncan studied astronomy at Indiana University, Bloomington, and earned an A.B. degree in 1905 from the School of Mechanics and Astronomy.
Upon graduation, he was awarded a Lawrence Fellowship for students of astronomy, which entitled him to spend one year in the Flagstaff observatory in Arizona while he pursued a Masters degree. The fellowship was a result of Percival Lowell's search for Planet X. He hoped find it by means of a broad-brush photographic search of the sky along the ecliptic. For this purpose, John and two other Indiana University graduates were assigned to photograph the sky. By the time the fellowship was discontinued, in September 1907, no planet had been found.
Based on his work at Lowell Observatory, he received his Master's Degree in 1906 and taught for a year in Indiana University.
In 1907, he enrolled at the University of California, studying under a fellowship at Lick Observatory. He received a Ph.D. in 1909. His dissertation was published as “The Orbits of the Cepheid Variables Y Sagittarii and RT Aurigae; with a Discussion of the Possible Causes of This Type of Stellar Variation,” in Lick Observatory Bulletin, 5 (1908–1910), 82–94, written under the direction of William Wallace Campbell.
Duncan's educational experience provided an example for a successful contemporary doctoral candidate. Writing to Campbell concerning a fellowship to work on a Ph.D. at Berkeley, Duncan included a summary of his undergraduate and graduate work at Indiana. This included:
* Fifty-two hours in astronomy including descriptive astronomy, celestial mechanics, theory of orbits, least squares and practical observatory work with a 12-inch refractor, transit circle and photographic camera, as well as a 15-inch reflector.
* Thirty hours of physics, including spectroscopy using a 4-inch diffraction grating ruled in the laboratory of Henry Rowland at Johns Hopkins.
* Twenty-eight hours of mathematics through differential equations.
* An introductory chemistry course
* Extensive work in French and German.
* English literature and composition.
* A master's degree on the strength of his experience at Lowell Observatory.
Duncan had studied celestial mechanics using the latest texts by University of Chicago professor Forest Ray Moulton (1872-1952) and Félix Tisserand (1854-96), of the Paris Observatory.

## Career

Duncan’s chief contribution to astronomy was his photographic demonstration of expansion in the Crab nebula. He is perhaps better known, however, as the author of Astronomy, a standard college textbook for over thirty years, which was illustrated with many of his own photographs of nebulae and galaxies.
In the 1900 US Census, 18 year old John C. Duncan is listed among family members as a salesman (perhaps for the flour mill) and a "comer" (upstart), perhaps reflecting his recent high school graduation.
During his undergraduate studies 1901 to 1903, Duncan was also a country school teacher in Indiana. Duncan taught at Indiana University in 1904-05 and acted as and observatory assistant in the Department of Mechanics and Astronomy. Dr. Duncan subsequently spent a year at Flagstaff at Lowell Observatory as a Lawrence Fellow. He was also a fellow at the Lick Observatory from 1907 to 1909.
After receiving his doctorate, he returned to the East, serving as instructor in astronomy at Harvard University from 1909 to 1916, and Radcliffe College, 1911-16. He became professor of astronomy and director of Whitin Observatory at Wellesley College in 1916, taking over from Dr. Sarah Frances Whiting upon her retirement, positions he held until his own retirement from full time work in 1950.
In 1923, reflecting increased awareness of the need to integrate spectroscopy early on in the astronomy curriculum, Dr. Duncan together with Professor H.T. Stetson issued an astronomical laboratory handbook that included a short discussion of the ‘‘principles of spectroscopy,’’ grating and prism spectroscopes and the classification of stellar spectra. This prepared students for the succeeding specialized course in Elementary Astrophysics also taught by Stetson every other year.
The underlying reason for this shift in the curriculum was the rise in spectroscopy theory alongside quantum theory and quantum mechanics. In addition to rotational and relative stellar velocity, spectra became indicators of electron transitions between states whose energy level could be precisely calculated. This provided the basis for the transition of new astronomers from measurement takers to physicists at their core.
Duncan finished his career as a visiting professor at the University of Arizona, a position he held until 1962.
He was a visiting astronomer at Mount Wilson Observatory in 1920 to 21, and a summer voluntary research assistant there from 1922 to 1949. Among the many images he collected during his initial time with the 100 inch Hooker Telescope, his photograph of the Horsehead Nebula was long considered the definitive example, and he apparently was first to coin its popular name in the first edition of his astronomy textbook in 1926.
For his June 27, 1919 photograph of the Eagle Nebula, he provided the first published description of the structures now known as the Pillars of Creation.
His many research papers included his work on the Crab nebula appeared as “Changes Observed in the Crab Nebula in Taurus,” in Proceedings of the National Academy of Sciences, 7 (1921), 179–180; and as “Second Report on the Expansion of the Crab Nebula,” in Astrophysical Journal, 89 (1939), 482–485. Reproductions and descriptions of the best of Duncan’s photographs are contained in six papers: “Bright Nebulae and Star Clusters in Sagittarius and Scutum,” in Astrophysical Journal, 51 (1920), 4–12, with 4 plates; “Bright and Dark Nebulae near ζ Orionis, Photographed with the 100-inch Hooker Telescope,” ibid., 53 (1921), 392–396, with 2 plates; “Photographic Studies of Nebulae, Third Paper,” ibid., 57 (1923), 137–148, with 11 plates; “Photographic Studies of Nebulae, Fourth Paper,” ibid., 63 (1926), 122–126, with 4 plates; “Photographic Studies of Nebulae, Fifth Paper,” ibid, 86 (1937), 496–498, with 6 plates; and “Photographic Studies of Nebulae VI. The Great Nebulous Region in Cygnus Photographed in Red Light,” ibid., 109 (1949), 479, with 2 plates.
Dr. Duncan's career was a fruitful combination of research and teaching both in his own classrooms and through his book.

## Discoveries
### Variable stars in distant galaxies

Using the 100-inch and 60-inch Mount Wilson Observatory telescopes, Duncan, by then director of the Wellesley College Observatory, first spotted a variable in what was then called the spiral nebulae M31 in 1922, and three variables within the area covered by the M33 nebulae in 1925. In 1926 Edwin Hubble was able to determine that these stars were classical Cepheids, which allowed him to estimate their distance. The results of this supported the concept that spiral nebulae were galaxies in their own right, rather than clouds within the Milky Way.

### Expansion of the Crab Nebula

Dr. Duncan compared photographic plates taken 11.5 years apart, and found that the Crab Nebula was expanding at an average of about 0.2 seconds of arc per year, beginning roughly 900 years before.

### Supernova spectroscopy

In the four decades from 1896 to 1936 only three supernovae were observed spectroscopically: George W. Ritchey, with the assistance of Francis Pease, observed a faint supernova in a spiral galaxy, NGC6946, in 1917 and obtained an objective prism plate. In 1926 another faint supernova in a spiral system was observed both at Lick Observatory by Donald Shane and at Mt Wilson by Duncan and Seth Barnes Nicholson.

## Personal life

Duncan married Katharine Armington Bullard on Dec. 29, 1906. They had one daughter, Eunice Naomi.
Katharine was born on 31 August 1879, in Indianapolis, graduated from a Quaker secondary academy, and from the normal college in Flagstaff, Arizona. During her senior year in Flagstaff, she met her future husband, tutoring him in Spanish. She died on 21 September 1952, in Tucson, Arizona at the age of 73.
Upon his death in 1967, John C. Duncan's survivors included his daughter, two grandchildren, and three great grandchildren.

## Honors

Duncan was a Fellow of the Royal Astronomical Society and the American Astronomical Society (as Secretary of the later from 1936 to 1939), as well as a member of the International Astronomical Union, and since 1938 the American Association for the Advancement of Science, the American Academy of Arts and Sciences and numerous other scientific organizations. The asteroid 2753 Duncan, discovered on 18 February 1966 at the Goethe Link Observatory, was named after him.