At the University of Chicago he was associate in astronomy (1898–1900), instructor (1900–03), assistant professor (1903–08), associate professor (1908–12), and professor after 1912.
Di Universitas Chicago ia menjadi associate bidang astronomi (1898-1900), instruktur (1900-03), asisten profesor (1903-08), associate profesor (1908-12), dan profesor sejak 1912.