(born Nov. 20, 1889, Marshfield, Mo., U.S.—died Sept. 28, 1953, San Marino, Calif.) U.S. astronomer. He earned a degree in mathematics and astronomy at the University of Chicago, then made a brief foray into law before returning to astronomy. After earning his Ph.D., he began working at Mount Wilson Observatory. In 1922–24 he discovered that certain nebulae contained Cepheid variable stars; he determined that these were several hundred thousand light-years away (outside the Milky Way Galaxy) and that the nebulae they were in were actually other galaxies. In studying those galaxies, he made his second remarkable discovery (1927): that the galaxies were receding from the Milky Way at rates that increased with distance. This implied that the universe, long considered unchanging, was expanding (see expanding universe); even more remarkable, the ratio of the galaxies' speed to their distance was a constant (see Hubble's constant). Hubble's original calculation of the constant was incorrect; it made the Milky Way larger than all other galaxies and the entire universe younger than the surmised age of Earth. Later astronomers determined that galaxies were systematically more distant, resolving the discrepancy.

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He worked with William Pickering at Lowell Observatory in 1937 creating the first color photographs of Mars. He then worked at the Dearborn Observatory from 1939 until 1941. During the Second World War he was an optical engineer for the U.S. Army and worked on a tracking system for missiles using telescopes. After the war he continued to work on this field until 1958. He then became a research physicist at the Scripps Institute, and later accepted a position at the Jet Propulsion Laboratory in 1960. There he was responsible for the cameras on the Ranger spacecraft exploring the Moon. At the time of his death he had worked out the preliminary design for the camera system of the Voyager space mission.

He received the George W. Goddard Award in 1964.

A crater on Mars was named in his honor.

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