Late in the evening of October 5, 1923, Edwin Hubble sat at the eyepiece of the Hooker telescope at the Mount Wilson Observatory, atop the mountains overlooking the Los Angeles basin. He was observing an object in the northern sky. To the unaided eye, it was visible as a faint smudge. But through a telescope it sharpened into a brilliant ellipse called the Andromeda Nebula. To settle a debate...