American astronomer whose systematic observations (1912–25) of the extraordinary radial velocities of spiral galaxies provided the first evidence supporting the expanding-universe theory.

Born on an Indiana farm, Slipher studied at Indiana University (B.A., 1901; M.A., 1903; Ph.D., 1909). In 1901 he joined the staff of the Lowell Observatory at Flagstaff (though he returned to Indiana at times for graduate study), and he became its acting director in 1916 and director in 1926. There he organized and guided the search that resulted in the discovery of Pluto in 1930. Slipher's extensive investigations led to the determination of the rotational periods of several planets. His discovery of dark absorption bands in the spectra of Jupiter, Saturn, and Neptune led to the identification of some of the chemical constituents of their atmospheres. He demonstrated that many diffuse nebulae (clouds of dust and gas) shine by the reflected light of nearby stars and discovered the bright radiations of the night sky and their changes in intensity. He also proved that sodium and calcium are scattered throughout interstellar space.