The Helliconia Trilogy

Helliconia follows an eccentric orbit around a double-star system with a twenty-six-hundred-year cycle of very long seasons. As spring slowly breaks the brutally long winter, humans emerge from hiding and a long sequence of civilization and growth begins to repeat again, unbeknownst to the participants but watched by an orbiting satellite station, Avernus, created by Earth some centuries ago. Humans free themselves from slavery to the aboriginal Phagors, and religion and science flower and expand.

A handful of centuries on, Helliconia is close to the larger star in its binary system, and the Phagors have been driven into exile, but conflicting religions and hostility to science keep human civilization fragmented and constantly fighting wars over petty power and fertile land as a plague devastates populations. However, everything changes when a secret visitor from the observer satellite from Earth accepts a slow death in order to visit the planet and spend his time in the sunlight and open air.