He was on holiday in the Netherlands when the First World War broke out. Prevented from returning to Germany, he started work as a cosmography and science teacher. Though the Leidse Sterrewacht wanted him back, government opposition because of his Marxist sympathies prevented his return. Instead, the Amsterdam city council appointed him at the University of Amsterdam in 1918 as a lecturer. In 1921, he founded the astronomical institute there; in 1925, he was appointed as extraordinary professor; and in 1932 as full professor.