On Christmas Eve 1968, the three astronauts of Apollo 8 became the first humans to orbit the Moon. On their fourth orbit, as the spacecraft came around from the far side, Earth rose over the lunar horizon. The crew scrambled for cameras, and William Anders took this photograph.

They had gone to look at the Moon. What stopped them was looking back.

The image arrived at the end of a brutal year — assassinations, war, protest — and it gave people an unexpected perspective: one small, fragile, beautiful world, with no borders visible from space. It is often credited with helping inspire the modern environmental movement; the first Earth Day was held in 1970.

It is also a reminder that the 1960s weren't only about flowers and music. They were about a strange new sense of the planet as one place.