The story of the Apollo 17 Blue Marble: the single photograph of the whole Earth taken by the last people ever to leave Earth's orbit, shot on 7 December 1972 with a hand-held Hasselblad 500EL — and what the image teaches a player about dating a 1972 photograph.
The Blue Marble is a photograph of the entire Earth as a sunlit blue-and-white "disc" hanging in black space, taken by the Apollo 17 crew on 7 December 1972 from roughly 29,000 kilometres out, on their way to the Moon. It is one of the most reproduced photographic images in history: a single frame broadcast and reprinted so often that a cropped, rotated version of it has become the default picture people carry of planet Earth itself. It arrived at the exact moment the environmental movement was finding its first Earth Days, and it became the image that movement put on its banners — even though the crew, as they shot it, had no idea how far the frame would travel.
Apollo 17 launched on 7 December 1972 with Gene Cernan, Ronald Evans and Harrison Schmitt aboard — the final Apollo lunar mission, and a launch date that carried a quiet symmetry the crew never needed to point out: exactly thirty-one years to the day after the attack on Pearl Harbor. About five hours into the flight, coasting toward the Moon, the Earth behind them sat fully lit by the Sun, and the mission's trajectory had finally worked out so that the crew could see, for the first time from a crewed flight, the Antarctic ice cap at the bottom of the world. Cernan and Schmitt went on to be the last two humans to walk on the Moon; Evans the last to orbit it. When Apollo 17 splashed down on 19 December 1972, no person had been that far from Earth since — and none has been since. No human being has seen the whole Earth as a disc from beyond our orbit in the half-century since that crew did.
The shot was made with a modified Hasselblad 500EL electric camera, an 80mm Zeiss Planar lens set at f/2.8, a shutter speed of 1/250 of a second, Kodak SO-368 colour film, and — because the 70mm back had no viewfinder — a good deal of aiming by feel. The famous image is catalogued as AS17-148-22727, one of a short bracketed series taken in a few minutes. Who actually pressed the shutter remains one of the great unresolved authorship questions in photography. NASA credits the whole crew. Gene Cernan claimed it; a TIME piece in early January 1973 credited Ronald Evans; Harrison Schmitt claimed it as well; and the investigator Eric Hartwell, reconstructing the sequence from the flight transcripts, concluded that the evidence most supports Schmitt. Even here — in the most reproduced photograph in history — the record refuses to state with confidence who took it. That is the provenance lesson the archive is built on: you report what the source actually supports, and the source here is "the crew of Apollo 17". On a guessNow reveal the same thing happens — the attribution shown is the one the record gives, not a fact we wish it gave.
Part of why the authorship is fought over is that the frame exists at all is bound up with Schmitt's private hobby. Before NASA he had been an amateur meteorologist, and aboard Apollo 17 his personal project was to photograph the day's weather from orbit and try to forecast the next day's from what he saw. What he wanted in the picture was not chiefly the whole planet but the weather system over India — a large cyclone that real records confirm was churning in the Indian Ocean from the start of December 1972, and that you can see curling off the top of Africa in the frame. The version of the image the world knows is a crop and rotation of the original, which had the South Pole uppermost; the archive keeps the original orientation with the credit, licence and source shown beside it — the same provenance discipline guessNow applies to every reveal.
For a guessNow player this is one of the cleanest dating problems in the archive, because the frame itself traps the date from both ends. A hand-held photograph of the whole Earth as a fully lit disc, taken from beyond Earth orbit, could not exist before humans flew beyond low Earth orbit — which began with the Apollo programme in December 1968 — and could not exist after humans stopped going, because Apollo 17 was the last crewed mission past Earth orbit and no such image has been made since it returned on 19 December 1972. Inside that bracket the particular view — the whole African coastline, the Arabian Peninsula, Madagascar, the pole cap, the Sun directly behind the camera — matches the single translunar coast of early December 1972. The discipline the game trains is exactly here: read the newest thing in the frame (a viewpoint only astronauts can hold) and the last thing (no one has had this view in fifty years), and a player who cannot name the year can still bracket the moment to within days. For the wider mid-century cues, our how to date a 1970s photograph guide and the general how to date an old photograph work through the tells in detail.
The photograph is one of the rounds in guessNow's archive. It arrives revealed piece by piece, and you slide to the year you think it was taken before your HP runs out — can you land the whole Earth on 1972? Read first from the newest thing in the frame, then from the last thing, and see whether you can pin the decade and the year the way a careful player can.