Lucian of Samosata’s True History (~2nd Century)
The Syrian author Lucian was both a very pragmatic person and a very sarcastic writer, and ludicrous tales of myths and fantasy presented as facts annoyed him very much. As a result, he decided to write his own. True History is an utterly fallacious account of Lucian’s travels to a far-off continent, a sea of milk, inside a whale, and even the moon. There, he discovered a war between the animal-human denizens of the moon and the sun, thanks to giant spiders who tether the two spheres with their webs. The moon people also have detachable genitals, so you probably couldn’t classify this as hard sci-fi per se, but it shares plenty of similarities with Edgar Rice Burroughs’ John Carter of Mars books.
