"The Earth reminded us of a Christmas tree ornament hanging in the blackness of space. As we got farther and farther away it diminished in size. Finally it shrank to the size of a marble, the most beautiful marble you can imagine. That beautiful, warm, living object looked so fragile, so delicate, that if you touched it with a finger it would crumble and fall apart."
This quote comes from James Irwin, Apollo 15. Many of the astronauts have commented on how fragile the Earth appears, but I really liked the analogy to the Christmas tree ornament, maybe because I have broken enough of them in my day to understand just how fragile they are.
Countdown to Earth Day part 1