 From 1903 to 1905, he worked at the Cambridge Observatory with Arthur Robert Hinks as a research assistant of the Carnegie Institution and came under the strong influence of George Darwin.He returned to Princeton to become an instructor in astronomy (1905–1908), assistant professor (1908–1911), professor (1911–1927) and research professor (1927–1947).He was also the director of the Princeton University Observatory from 1912 to 1947 where Charlotte Moore Sitterly helped him measure and calculate the properties of stars.