of Pluto in 1930.

By 1917, Slipher had measured the radial velocities of 25 "spiral nebulae," and found that all but three of those galaxies were moving away from us, at substantial speeds. Slipher himself speculated that this might be due to the motion of our own galaxy - as in his sample, those galaxies moving towards us and those moving away from us were roughly in opposite directions. In hindsight, this was the first data supporting models of an expanding universe.