In its early days, photography was, at best, a second thought at NASA. John Glenn had to convince NASA to let him take a camera with him on his historic mission to orbit the Earth aboard Friendship 7. Six years later, Apollo 8 carried a Hasselblad 500EL stripped down to save weight and equipped with customized film, lubricants, and lenses. The mission plan called for the crew to take photos of the moon's surface, not the Earth.
"Wow, is that pretty," Bill Anders said, watching the moon rise on Apollo 8's fourth lunar revolution. Mission Commander Frank Borman said, half-jokingly, "Don't take that; it's not scheduled," but Anders started snapping photos anyway, disregarding the tight schedule for the day. Fighting the fleeting view of the moon and fogged windows, Anders switched to color film and took one of the most famous photos in history. This video tells the story beautifully.
Even after the mission's conclusion, NASA didn't appreciate the photo's significance. The iconic photograph is not even listed under "items of interest" in the "photographic tasks" section of the press kit for the mission.
Previously: NASA re-creates historic earthrise photo on Apollo 8 anniversary, in video form