Kuiper finished his doctoral thesis on binary stars with Hertzsprung in 1933, after which he traveled to California to become a fellow under Robert Grant Aitken at the Lick Observatory. In 1935 he left to work at the Harvard College Observatory, where he met Sarah Parker Fuller, whom he married on 20 June 1936. Although he had planned to move to Java to work at the Bosscha Observatory, he took a position at the Yerkes Observatory of the University of Chicago and became an American citizen in 1937.