At the end of World War I, Edwin Hubble held the rank of major. He served in the United States Army and rose rapidly in rank during his wartime service.
Hubble completed his doctoral work at the University of Chicago in 1917, shortly after the United States entered the war. He then volunteered for military service and was assigned to the 86th Division. By 1918, he had reached the rank of major and was considered fit for overseas duty. He served with the American Expeditionary Forces in France before the war ended.
The military episode interrupted Hubble’s planned move into professional astronomy. Before the war, George Ellery Hale had invited him to join the staff at Mount Wilson Observatory in California. After Hubble’s discharge in 1919, he finally took up that position. His military experience became an important but relatively brief chapter between his doctoral studies and his groundbreaking astronomical career.
A frequent source of confusion is Hubble’s later military service during World War II. In that conflict he worked as a civilian at Aberdeen Proving Ground, where he directed research in exterior ballistics. That later role should not be confused with his World War I rank: during World War I, he was a major.