It was on this day in 1972 that astronauts on the Apollo 17 spacecraft took a famous photograph of the Earth, a photo that came to be known asThe Blue Marble.” Photographs of the Earth from space were relatively new at this time.

On Christmas Eve of 1968, the astronauts on the Apollo 8 mission, orbiting the moon, took a photo with the gray, craggy surface of the moon in the foreground and the bright blue Earth coming up behind, only half of it visible. That photo was called “Earthrise,” and it really shook people up because it made the Earth look so fragile, and because the photo was taken by actual people, not just a satellite.

And on this day in 1972, the crew of Apollo 17 took another photograph, not only one of the most famous images of the Earth but one of the most widely distributed photos ever taken. It’s known as “The Blue Marble” because that’s how the Earth looked to the astronauts. It was the first clear photo of the Earth, because the sun was at the astronauts’ back, and so the planet appears lit up and you can distinctly see blue, white, brown, even green. It became a symbol of the environmental movement of the 1970s, and it’s the image that gets put on flags, T-shirts, bumper stickers, and posters.

The crew of Apollo 17 was about 28,000 miles away from Earth when they took the Blue Marble photo. It was the last time that astronauts, not robots, were on a lunar mission — since then, no people have gotten far enough away from the Earth to take a photo like it.