The astronauts had spun around the moon a few times already, their gaze pointed down on the gray, pockmarked lunar surface. But now as they completed another orbit of the moon on Christmas Eve 1968, Frank Borman, the commander of the Apollo 8 mission, rolled the spacecraft, and, soon, there it was. Earth, this bright, beautiful sphere, alone in the inky vastness of space, a soloist at the edge of the stage suspended in the spotlight. “Oh, my God,” exclaimed Bill Anders, the lunar module pilot. “Look at that picture over there! There’s the Earth coming up. Wow, is...