Vesto Melvin Slipher (1875-1969), US astronomer. Slipher used spectroscopy to study planets and galaxies, using the Doppler effect to determine their motions. His most important work was discovering that the Andromeda Galaxy was approaching Earth at 300 kilometres per second, the fastest velocity recorded at that time (1912). As well as this, by 1917 he had found the velocities of 14 other galaxies, of which 13 were receding from Earth. These findings paved the way for the classification of galaxies as huge star systems far outside our Milky Way galaxy, and also for the expanding universe theory, later confirmed by Edwin Hubble.