Which U.S. military branch did Edwin Hubble serve in during World War I?

Edwin Hubble served in the United States Army during World War I.

When the United States entered the war in 1917, Hubble rushed to complete his doctoral work and joined the Army. He became a captain in the 343rd Infantry Regiment of the 86th Division and served in France, later attaining the rank of major.

Hubble’s military service interrupted the scientific career that would soon make him famous. After returning to civilian life in 1919, he accepted a position at Mount Wilson Observatory in California. There he used the powerful Hooker Telescope to study galaxies and helped establish that the universe extends far beyond the Milky Way.

The answer is sometimes muddled by modern military categories. The U.S. Air Force did not yet exist as an independent branch during World War I; military aviation was then organized within the Army. Hubble’s documented service was with the Army’s infantry, not the Navy, Marines, or a separate Air Force.