An astronomer measured galaxies racing away before anyone knew what a galaxy was
Starting in 1912, Vesto Slipher spent countless cold nights at Arizona's Lowell Observatory measuring the spectra of what were then called 'spiral nebulae.' He found that most were redshifted, meaning they were speeding away from Earth at enormous velocities. Those measurements became the raw data Edwin Hubble later used to show the universe itself is expanding.