When Vesto Slipher was born, it was commonly accepted that the universe was composed of a single galaxy. Everything that was visible, whether a star or a mysterious, wispy nebula, was a part of the Milky Way.

This opinion had not altered in the twenty-five years following Slipher's birth. When photographed, some of these nebulas showed a spiral configuration, and Astronomer Percival Lowell (1855-1916) felt certain the spiral clouds were planetary systems that were being formed. He hoped that understanding these spirals would increase the knowledge about the formation of our own solar system. Lowell needed someone to perform the study and, in 1901, he hired Slipher to work at the Lowell Observatory.

The process was a simple one: study the spectrum of the light from the nebulas and measure any Doppler shift present. If the light of the nebula was shifted toward the blue end of the spectrum...