“Hey, don’t take that, it’s not scheduled,” Frank Borman said, joking to his fellow Apollo 8 astronauts, Bill Anders and James Lovell, on Dec. 24, 1968. They were orbiting the moon, farther from Earth than any humans had ever been. On the fourth pass, they were confronted by an extraordinary sight that jolted them out of their regimented procedures. There, seen through a small window, was Earth itself, rising out of the void.

For a split second, the astronauts were dazzled by the luminescent blue sphere, whorled by a white cloud cover. Then, as they were trained to do, they went back to work. As it turned out, Mr. Anders was the one who snapped a color photo, just after his fellow astronauts, Frank Borman and James Lovell, called his attention to the greatest photo op in history.