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Doctoral studies

After the United States declared war on Germany in 1917 during World War I, Hubble rushed to complete his Ph.D. dissertation so he could join the military. Hubble volunteered for the United States Army and was assigned to the newly created 86th Division, where he served in the 2nd Battalion, 343rd Infantry Regiment. He rose to the rank of major, and was found fit for overseas duty on July 9, 1918; the 86th Division moved overseas, but never saw combat as it was broken up and its personnel used as replacements in other units. After the end of World War I, Hubble spent a year at Cambridge University, where he renewed his studies of astronomy.