 After receiving his Master of Arts in 1906, he began his doctoral studies at the University of California under the director of the Lick Observatory William Wallace Campbell, and in 1909 defended his dissertation on the Cepheids Y Sagittarii and RT Aurigae.Duncan was a lecturer at Harvard University from 1909 to 1916, and from 1911 until 1916 also at Radcliffe College, before he was appointed in 1916 the professor and director of the Astronomy Department of Wellesley College and director of the Whitin Observatory.After retiring in 1950, he spent the next twelve years as a visiting professor at the Steward Observatory.