The Milky Way, just like every galaxy in the cosmos, moves. While everything in the universe is constantly moving because the universe itself is expanding, since the 1970s astronomers have known of an additional motion, called peculiar velocity. This is a different sort of flow that we seem to be caught in. The Local Group of galaxies — a collection that includes the Milky Way, Andromeda and a few dozen smaller galactic companions — moves at about 600 kilometers per second with respect to the leftover radiation from the Big Bang. Over the past few decades, astronomers have tallied up...