Helliconia

The Helliconia Trilogy, by Brian Aldiss, is an epic sci-fi series that follows the lives of the Human Aliens that live on the titular planet. The trilogy's timeline stretches thousands of years, following certain influential characters as civilizations rise and fall.

Set in a binary star system, Helliconia has "years" of two different lengths. The "Small Year", about 480 days, is how long it takes for Helliconia to orbit its main star Batalix. The "Great Year", in which Helliconia and Batalix both orbit the much larger star Freyr, is over 1,800 Small Years.

Helliconia spends most of the Great Year as an ice planet, as Batalix is much dimmer than Earth's sun and its highly eliptical orbit means that Freyr is too distant to provide any warmth. As the Great Year proceeds, though, the much larger and brighter Freyr comes into play, causing gradual but severe climactic changes. As such, Helliconia goes though extended "seasons" that lasting centuries.

Despite being relatively Earth-like, Helliconia is host to multiple intelligent species, the most prominent of which are Humans and the Phagors, mankind's primary antagonist.

The series picks up as Helliconia is transitioning from it's long Winter into Spring. As the planet begins warming up and becomes more hospitable, mankind begins to develop civilization and become dominant in the world.

The trilogy comprises these books:
 * Helliconia Spring
 * Helliconia Summer
 * Helliconia Winter

This series provides examples of:

 * A Load of Bull: The Phagors, the other sentient race of Helliconia and constant antagonists to mankind, resemble minotaurs.
 * Alien Sky: As Helliconia is in a binary star system, this is a given. Brian Aldiss goes to great lengths to depict the movements of the stars in Helliconia's sky accurately, including how the orbital dynamics of the Helliconia-Batalix-Freyr system would play out from the perspective of Helliconia.
 * Batalix, appearing larger in the sky, actually eclipses Freyr several times. Think about that for a second -- imagine our sun being eclipsed by another sun.
 * Bizarre Alien Biology:
 * The Phagors have inverted organs; their intestines are above their lungs, among having yellow blood.
 * The Hoxneys, resembling zebras or horses, start life as crystalline beings.
 * There's also the Barsimmip Trees -- which are actually very large, subterranean, and hollow.
 * Wutra's Worm -- A gargantuan worm with a lifespan as long as the Great Year. Goes through several drastic metamorphises as the Great Seasons pass; at one point, a Worm will actually split into two smaller worms with wings, which then take to the sky.
 * Bolivian Army Cliffhanger: The first book ends as The reader is Left Hanging, since the second book begins in a completely seperate country, with new characters. Made even worse as the reader soon finds out
 * Call a Rabbit A Smeerp: Averted, since the various non-obviously-alien animals in the series are described pretty much as you would expect. There are sheep, pigs, and even terrestial plants such as wheat. However....
 * Call a Smeerp A Rabbit: ...they're never described in enough detail to tell either way.
 * Further confused by the obviously-alien-looking creatures and fauna of Hellliconia that exist alongside the more recognizable ones (with unique obviously-alien names to boot).
 * Dumb Is Good: In the first novel, the major philosophy of Aoz Roon, leader of human village where the plot takes place. He discourages too much learning, since he doesn't want to have any "idle mouths" to feed (i.e. teachers, philoshopers, etc)
 * Eternal Recurrence: The Great Year and it's great winter which always plunges humanity back into the stone age.
 * Fantastic Racism: Between the humans and phagors; both war with and enslave each other every chance they get.
 * Horse of a Different Color: The "hoxneys", which resemble colorful zebras.
 * Human Aliens: The humans of the series. Described in nearly every way as human, yet they apparently are native to Helliconia.
 * Human Subspecies: Several. Tend to be treated horrible by both humans and phagors. One example is the "Maqi", hobbit-like creatures with 8 fingers on each hand and "keen eyesight" -- they're most commonly depicted as slaves.
 * Humanoid Aliens: Phagors, the other intelligent species on Helliconia. Look like a cross between a yeti and a minotaur.
 * Imported Alien Phlebotinum: At the start of the second book, a dead man washes ashore a distant beach with a digital watch hidden on his person.
 * Instant Ice Just Add Cold: An important plot point in the first novel, when a woman believed to be a sorceress seemingly flash-freezes a group of charging phagors.
 * Lost Technology: Due to the cyclical nature of civilization, the humans of Helliconia, despite starting at a near-stone age level, possess numerous devices that are much more advanced than they are capable of manufacturing, such as fine clocks or telescopes.
 * Maybe Magic Maybe Mundane: The human "gossies" and "fessups"; ancestral spirits seemingly able to be contacted by entering a trance. Despite this one possibly supernatural element, the rest of the series is firmly in the "mundane" category.
 * The "air octaves" and "land octaves" often mentioned in the books are used for various seemingly supernatural abilities, and yet are described suspiciously similar to magnetic fields.
 * Mundane Dogmatic: Despite being set on an alien world, the series reads more like a Fantasy novel (just without magic) due to the primitiveness of the human society and the novel's focus on the characters.
 * The Plague: Strikes cyclically, twice every Great Year (early in the spring and then in late fall). The first, "Bone Fever", kills half of who it afflicts, and those that survive lose at least a third of their body weight (and keep it off, as anorexia is a permanent side-effect). The second is correspondingly called "The Fat Death", which makes you gorge on anything edible, including your fellow persons.
 * Actually the virus that induces these plagues is symbiotic with Helliconia's humans; they couldn't survive there otherwise. Only very fat people can get through the long, bitter winter and only skinny ones can survive the blistering summer.
 * The virus is also lethal to non-Helliconians (i.e. Earthlings)
 * Planetary Romance
 * Precursors: Humans themeselves. Humanity goes through the cycles as the "great years" pass, emerging from ignorance, building up, becoming relatively advanced, then falling back into war, ignorance, and eventually pitifulness again when the great winter falls. Cities rise up in the same location where they once were, since the ruins from past cycles are still standing (or at least the foundations provide an outline).
 * Prolonged Prologue: In the first novel, the lengthy opening details the life and times of Yuli, a man who goes on to found the village where the plot takes place. The plot itself actually starts hundreds of years afterward, and has nothing to do with Yuli.