US astronomer and mathematician who published a catalogue of nearly 3,000 radial velocities of stars 1928. His spectroscopic observations of Nova Auriga 1892 enabled him to describe the changes in its spectral pattern with time.

Campbell was born in Hancock, Ohio, and studied at the University of Michigan. From 1891 he worked at the newly established Lick Observatory, California, becoming director of the observatory 1901-30. There he was responsible for much of the spectroscopic work undertaken and was an active participant in and organizer of seven eclipse expeditions to many parts of the world.
In 1896 Campbell initiated his lengthiest project, the compilation of a vast amount of data on radial velocities. Useful for the determination of the motion of the Sun relative to other stars, the programme also led to the discovery of many binary systems, and the data were later used in the study of galactic rotation.
Campbell also confirmed the work done in 1919 by English astronomer Arthur Eddington on the deflection of light during an eclipse, which supported the general theory of relativity. The positive result Campbell obtained in 1922 was arrived at only after two previous attempts (1914 and 1918) had been frustrated by poor weather conditions and inadequate equipment.
Campbell went against the popular opinion of the time in reporting his observations on the absence of sufficient oxygen or water vapour in the Martian atmosphere to support life as found on Earth.