Forty years ago, the biggest TV audience in history tuned in to watch humankind's first close encounter with another world, as the crew of Apollo 8 reached lunar orbit. Here, the Apollo historian and film-maker Dr Christopher Riley gives his perspective on the mission and how that Christmas Eve of 1968 changed the world.
Back in 1948, the British astrophysicist Sir Fred Hoyle predicted that when spaceflight enabled us to see the whole Earth from space, the view would change us forever.
Hoyle would have to wait another 20 years before humans would get to see this view with their own eyes, when the crew of Apollo 8 became the first astronauts to leave Earth orbit.
By then, a handful of satellites had snapped a number of breathtaking portraits of the Earth from afar and even a stunning shot of the Earth rising above the Moon's surface from lunar orbit.
Oh, my God! Look at that picture over there! Isn't that something
Apollo 8 crew member
But on Christmas Eve 1968, none of the astronauts on board Apollo 8 were ready for the opportunity to witness their own Earthrise.
In all the months of training and preparation which had preceded the mission, no-one had thought to schedule an attempt for the crew to glimpse and record the most moving of sights, as their jewel of a home planet, suspended in the blackness of space, rose from behind the barren lunar horizon.
The blue marvel
For the first three orbits, preoccupied by the Moon and their latest TV broadcast, the spacecraft was not orientated to give them a chance to see the Earth.
But as Apollo 8 nosed its way back from the far side of the Moon for the fourth time, one of the crew spotted the view by chance from a window, his reaction captured by the on board tape recorder.
"Oh, my God! Look at that picture over there!" he exclaimed. "Isn't that something "
The stills were taken orbiting the Moon's equator, with north at the top
After a quick joke about the fact that it was not in their flight plan to photograph it, the crew abandoned protocol and scrambled to get a snap of the occasion with their stills camera.
The Hasselblad only had a black and white film magazine in, resulting in the image above - the first photograph of Earthrise taken by a human as he watched it happen.
But this first historic picture is rarely reproduced.
Not content with this first monochromatic image, the astronauts rushed to find a colour film, and Bill Anders managed to snap two more frames which became the choice of photo editors for the rest of history.
Quite who spotted the view and took the first black and white Earthrise image is the subject of debate among historians.
Some say it was Bill Anders (and claim the flight recording proves it); others have pointed to Frank Borman as the man behind the camera.
Nasa is planning to eventually post the recording on its Apollo 8 Flight Journal website, so you can decide for yourself.
Perhaps more surprising than the unexpected nature of this historic photo-op is that during the rest of the mission, whilst the crew did spend time photographing and commenting on views of the Earth, they did not think to film an Earthrise on their 16mm movie cameras.
These Data Acquisition Cameras, as they were called, were carried to record technical mission moments.
The Apollo 8 crew would use them to film each other messing about during quiet moments; so they weren't averse to deviating from their flight plan.
But it seems that they were too busy with their other scheduled tasks that Christmas Eve to film mankind's first Earthrise.
This stunning high-definition movie of an "Earthrise" was filmed by the Japanese space probe, Selene
The profound juxtaposition of such a vibrant, vulnerable, living blue and white marble of a planet rising from the stark grey lunar landscape was not captured on moving film until the Apollo 10 and 11 crews filmed it the following year.
By the fourth expedition to lunar orbit on Apollo 12, filming the Earth rising from behind the Moon had lost its magic and was only captured once through a slightly fogged window. It was the last Earthrise ever filmed during the Apollo era.
These images, along with hundreds of other still pictures taken of the whole Earth during Apollo's nine flights to the Moon, helped to drive the momentum of a burgeoning green movement during the 1970s.
They fuelled an awareness of the vulnerability of the Earth which still resonates with us today and shapes our behaviour, as Fred Hoyle predicted it would.
These images helped to drive the momentum of a burgeoning green movement during the 1970s
Only 24 human beings have ever laid eyes on a view of the whole Earth from space. But thanks to a new generation of missions carrying high-resolution cameras beyond Earth orbit, moving HD footage of the whole planet is now available for all of us to marvel at.
In 2007, the Japanese lunar mission Selene transmitted back the first movies of Earthrise from lunar orbit since Apollo.
Even more ambitious, a Nasa project called The Deep Space Climate Observatory (DSCOVR) was conceived at the end of the 20th Century.
Its mission was to stream a continuous live colour image of the Earth from a million miles out in space. The data from DSCOVR was designed to help with modelling climate change.
The first picture of the whole Earth, taken by Lunar Orbiter V
Al Gore, who had supported the project from its conception, also suggested that such live footage of the whole Earth broadcast continuously over the internet would provide a powerful modern reminder of the fragility of our home planet - in the way that those first hand snapped Apollo pictures had done all those decades earlier.
"Goresat" as it was dubbed was never launched under the Bush presidency.
Some suggest that DSCOVR would not have helped the cause of an administration committed to a path of oil dependency.
Today, the satellite still rests in storage at Nasa's Goddard Space Flight Center in Maryland awaiting a more enlightened presidency to give it a green light.
Perhaps, under a new administration, and to mark the 40th anniversary of humankind's first vision of the Earth from space, that time has now come.
Dr Christopher Riley is the co-producer of the documentary feature film In the Shadow of the Moon and curates the online Apollo film archive project Footagevault
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