Vesto Melvin Slipher

Vesto Slipher joined Lowell Observatory in 1901 and stayed for fifty-three years, rising to its directorship. The observatory was founded by Percival Lowell to study Mars, but Slipher pointed its 24-inch refractor at fainter, less fashionable objects — the spiral nebulae that nobody yet knew were galaxies.

In 1912 he obtained a high-dispersion spectrum of the Andromeda nebula and measured a radial velocity of −300 km/s — the first Doppler measurement of a galaxy. Over the next decade he catalogued the radial velocities of dozens more, finding almost all of them receding from Earth at hundreds or thousands of kilometres per second. The pattern was unmistakable but the framework to interpret it did not yet exist.

When Edwin Hubble plotted those velocities against his 1924 distance estimates and found the linear law that now bears Hubble's name, the velocities on that plot were Slipher's. He did the hard observational work; others collected the glory. He also oversaw Clyde Tombaugh's discovery of Pluto in 1930 at the same observatory.