Nightfall
Robert Silverberg
 The fictional planet Lagash (Kalgash in the novel adaptation) is located in a stellar system containing six suns (Alpha, Beta, Gamma, Delta are the only ones named in the short story; Onos, Dovim, Trey, Patru, Tano, and Sitha in the novel), which keep the whole planet continuously illuminated; total darkness is unknown, and as a result so are stars outside the stellar system. A group of scientists from Saro University begins to make a series of related discoveries: Sheerin&nbsp;501, a psychologist, researches the effects of prolonged exposure to darkness, Siferra&nbsp;89, an archaeologist, finds evidence of multiple cyclical collapses of civilization regularly occurring approximately every two thousand years, and Beenay&nbsp;25 is an astronomer who discovered irregularities in the orbit of Lagash around its primary sun Onos. Beenay takes his findings to his superior at the university, Aton, who formulated the Theory of Universal Gravitation (the in-story discussion of same making light of an article once written about Einstein's Theory of Relativity, referencing the false notion that "only twelve men" could understand it). This forces the astronomers at Saro University to attempt to find an answer to what is causing this anomaly. Eventually it is discovered that the only thing that could be causing the deviation is an astronomical body that orbits Lagash. Beenay, through his friend Theremon&nbsp;762 (a reporter), has learned some of the beliefs of the group known as the Cult ("Apostles of Flame" in the novel). They believe the world would be destroyed in a darkness with the appearance of stars that unleash a torrent of fire. Beenay combines what he has learned about the repetitive collapses at the digsite, and the new theory with the potential of eclipses and concludes that once every 2049 years the one sun visible is eclipsed, resulting in a brief 'night'. Since the current population of Lagash has never experienced universal darkness, the scientists conclude that the darkness itself would traumatize the people and that the inhabitants of the planet would need to prepare accordingly. When nightfall occurs, however, the scientists (who have prepared themselves for darkness) and the rest of the planet are most surprised by the sight of hitherto-invisible stars outside the six-star system filling the sky. The short story does not dramatize events after darkness arrives, but in the novel and X&nbsp;Minus One program, civil disorder breaks out; cities are destroyed in massive fires and civilization collapses, with the ashes of the fallen civilization and the competing groups trying to seize control.