"Nightfall"[1] is a 1941 science fiction short story by the American writer Isaac Asimov about the coming of darkness to the people of a planet ordinarily illuminated by sunlight at all times.

He later said that before "Nightfall" neither he nor anyone else other than perhaps Campbell considered him more than a "third rater"; his name appeared on the cover of Astounding for the first time, and the story made him one of the field's top writers.

The researchers explain that they have discovered evidence of numerous ancient civilizations on the planet, all destroyed by fire, with the collapses occurring about 2,000 years apart.

Furthermore, the religious writings of a doomsday cult claim that Lagash periodically passes through an enormous cave where mysterious "stars" appear.

The scientists use this apparent myth, along with recent discoveries in gravitational research, to develop a theory about the repeated collapse of society.

A mathematical analysis of Lagash's orbit around its primary sun reveals irregularities caused by the presence of a previously undiscovered moon that cannot be seen in the light of day.

Psychological experiments involving darkness have revealed that Lagashians may suffer permanent mental illness or even death after as little as 15 minutes of exposure and the eclipse is projected to last for over half a day.

This, coupled with the diameter of the umbra being at least as great as that of the planet, ensures that the entire world population will experience an unprecedented period of prolonged, widespread darkness.

Oral accounts of the resulting chaos from crazed survivors and their children were passed down through the ages and formed the basis for the doomsday cult's sacred texts.

Present-day civilization is doomed for the same reasons, but the researchers hope that saving a few hundred sane people in a hideout lit by newly invented torches and conducting detailed observations of the upcoming eclipse will help to break the cycle of societal collapse.

Because of the perpetual daylight on Lagash, its inhabitants are unaware of the existence of stars apart from their own; Lagashian astronomers estimate that the entire universe is no more than a few light-years in diameter and may hypothetically contain a small number of other suns.

Learning that the universe is far vaster and Lagash far more insignificant than they believed, coupled with the worldwide darkness produced by the eclipse, drives everyone, including the scientists, insane.

Outside the observatory, in the direction of the city, the horizon begins glowing with the light of spreading fires as "the long night" returns to Lagash.