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I heard of this "hard" science fiction short story via word of mouth. Spoiler alert: my final sentences here will likely expose an evocative twist ending. My guess is that this story was written in the 1950's, possibly mid- to late-1950's. It takes place entirely on Earth. An astronaut who has not flown in space is inside a simulator or part of an experiment. The simulated mission is a multi-day trip from Earth to the vicinity of the Moon. The man in the fake-yet-realistic spacecraft can see the Moon slowly approaching. The image of the Moon may be a closed-circuit television image (large TV screen or projector) or the man is seeing, through a window, a large, fake-yet-realistic Moon globe. (Either way, the subject is actually looking at a fake Moon globe.) The near side of the Moon keeps getting bigger. Eventually the simulator gives the subject a glimpse of the far side of the Moon. Presumably, he is about to enter lunar orbit or swing around the far side of the Moon. The man then sees, for the first time, that the Moon is fake, and the large fake simulated Moon is not a complete sphere. The subject sees a makeshift structure (scaffolding) behind the realistic near side. He goes mad. Ending: the man keeps repeating the same few words: his description of the inelegant structure he sees "behind" the near side of the Moon. (It could be that during the simulation or experiment he actually believed he was in cislunar space, and his end-of-the-story ravings were a report to Earth of what the far side looked like, so perhaps he was only partially mad or not mad.) This story was almost certainly written before Gagarin flew, at a time when there were legitimate questions about human response to spaceflight. (By the way, Philip Zimbardo's multi-day mock prison experiment was around 1969-1972. After a few days, some subjects apparently thought they were in a real prison.) Perhaps someone could provide an author, short story title, or book title (collection of short stories).