Forty years ago this Christmas the first human beings reached the moon. But their historic feat is better remembered for an image of what they left behind - planet Earth.

Looking back from more than 200,000 miles away, the crew of Apollo 8 saw Earth floating "like a Christmas tree ornament lit up in space, fragile-looking". They pointed their cameras through smeared porthole windows and began snapping. It seems almost incredible now, but nobody thought to take a photo of Earth until they saw it, because nobody had seen it before.

One of those photos, an Earthrise over the grey and inhospitable lunar horizon, has become one of the most reproduced and recognised pictures of our planet. LIFE magazine selected it as one of 100 photographs that changed the world; more recently it featured in an Oscar-winning film about climate change, An Inconvenient Truth.

"That one picture exploded in the consciousness of humans," said Al Gore, the film's Nobel prize-winning narrator and campaigner. "It led to dramatic changes. Within 18 months of this picture the environment movement had begun."

There is still some dispute over which of Apollo 8's crew took the first Earthrise photo, but the official version selected by the American space agency, Nasa, was by Bill Anders, who spoke to the Guardian from his home in San Diego, California. "After all the training and studying we'd done as pilots and engineers to get to the moon safely and get back, [and] as human beings to explore moon orbit," he said, "what we really discovered was the planet Earth."

Anders and fellow crew members Frank Borman and Jim Lovell left on December 21 and began orbiting the moon on Christmas Eve. For the first and second loops, Apollo 8's crew faced backwards, but on the third revolution Borman, the commander, turned their capsule around. "Suddenly Borman said something like 'look at that' and here was the Earth coming up," recalled Anders. "There was a mad scramble for cameras: I just happened to have one with colour film in it and a long lens. All I did was to keep snapping.

"It's not a very good photo as photos go, but it's a special one. It was the first statement of our planet Earth and it was particularly impressive because it's contrasted against this startling horizon."

In the following weeks it is estimated that 2 billion people - more than half the humans alive at the time - watched the blurred black and grey TV film of the moon and listened to crackling voices speaking to them across space as Apollo 8's crew read the first 10 verses of the Bible.

In what now seems symbolic of the impact of seeing the whole planet for the first time with human eyes, Borman appeared to cast off the nationalistic cold war fervour surrounding the mission and ended the broadcast saying: "A merry Christmas and God bless all of you, all of you on the good Earth."

The impact of the photo in 1968 was immediate in a world already shaken by Rachel Carson's explosive book on pesticide pollution, Silent Spring. Four decades later, climate change is the great environmental threat and Earthrise is still used by campaigners trying to draw attention to the problem.

Landscape photographer Joe Cornish said he had been haunted by the image: "It's a new perspective from space, but it's a totally new perspective when you see it in relation to another body in space."

For Anders, the fragility of life on Earth is shown even more powerfully by three photos of the Earth alone in space: "Earthrise has a reference - there's the moon and the Earth, you don't get a vastness - whereas the other ones, particularly the smaller one, it's Earth and black to the frame ... it goes on and on."

He added: "I think it's important for people to understand they are just going around on one of the smaller grains of sand on one of the spiral arms of this kind of puny galaxy ... it [Earth] is insignificant, but it's the only one we've got."