"Nightfall" is a story about a planet that does not experience nightfall except once in every 2,049 years. With six suns, Lagash otherwise exists in perpetual sunlight. In the course of describing the last four hours before darkness covers all, Asimov explains how a rare eclipse is able to blot out all the light and why the event always results in universal chaos. This feat he achieves by placing the story in the Observatory of the scientists who are able to predict the coming phenomenon. Aton 77, the aged director of Saro University and chief astronomer, is preparing to try to record the eclipse and whatever follows so that there will be scientific evidence to explain what has happened.

On a planet where darkness is unknown, the expectation is that everyone will go insane from fear and claustrophobia, and that in their fear they will try to burn...