Americanspectroscopist, brother of E. C. Slipher. At Lowell Observatory he used spectroscopy to measure planetary rotation periods and atmospheric compositions. In 1912 he obtained the first radial-velocity measurements from the spectrum of a so-called ‘spiral nebula’ (the Andromeda Galaxy). By 1925 he had 45 radial velocities of other such nebulae, nearly all of which were receding too fast to belong to the Milky Way. This was the observational foundation for theories of an expanding Universe. In 1912 he discovered the existence of reflection nebulae from a study of the nebulosity around the Pleiades, proving that there is dust as well as gas in space. He supervised the photographic search that led to C. W. Tombaugh's discovery of Pluto in 1930.
Subjects: Astronomy and Astrophysics.