The Twelve Colonies of Kobol
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The Twelve Colonies of Kobol are twelve distinct worlds, located in a remote part of our Galaxy. For 2,000 years they were home to the descendants of humans participating in the exodus of Kobol. For most of the two millennia that these worlds were inhabited, the Twelve Colonies consisted of multiple sovereign states, but during the final fifty-two years leading up to the Fall of the Twelve Colonies, all twelve comprised a single state known as the United Colonies of Kobol (TRS: "Colonial Day").
After the settlement, the colonies slowly began to prosper and eventually gave birth to a race of cybernetic lifeforms, known as Cylons. Only a few years after their creation, the Cylons raged a devastating war on their creators' home planets, which ultimately led to the Fall of the Twelve Colonies. Wasted and in ruins, the once prosperous homeworlds of humanity were finally abandoned by both humans and Cylons.
The Twelve Colonies are situated within the star system Cyrannus, located some 2,000 light years from Kobol, where humanity presumedly first evolved. All in all, Cyrannus is a vast and complex system, comprised of four  stars: Helios Alpha, Helios Beta, Helios Gamma, and Helios Delta. Each star is orbited by its own family of planets and other astronomical bodies. The stars come together to form two binary systems (Alpha/Beta and Gamma/Delta). Both pairs of stars then orbit each other around a third, central barycenter every 546,892 years. Both binary systems are separated by 0.16 light years or 10,091 Stellar Units, shortened to SU (In this case, 1 SU being equal to the average distance between Helios Alpha and the barycenter of the Caprica-Gemenon binary, roughly 150 million km).
Helios Alpha is a main sequence star of spectral class G2V and has a mass of 1.0, and was the first system settled by the Colonials after the exodus from Kobol, having first set foot on Gemenon. It shares its orbit with Helios Beta at a distance of 63 SU. The pair complete an orbit once every 373 years.
This system is unique in the fact that Caprica and Gemenon share an orbit around a common barycenter. According to the Colonial map, the planets orbit each other once every 28.2 days, and are separated by a distance of 493,000 km. Based on these numbers, Kepler's Third Law learns us that the combined mass of these colonies is about twice that of Earth.
Helios Beta is a main sequence star of spectral class K1V and has a mass of 0.79. It was the second system settled by the Colonials after the exodus from Kobol. It shares its orbit with Helios Alpha at a distance of 63 SU. The pair complete an orbit once every 373 years.
Helios Gamma is a main sequence star of spectral class G9V and has a mass of 0.89. It was presumably the third system settled by the Colonials. Helios Gamma shares its orbit with Helios Delta at a distance of 70 SU, and completes an orbit once every 456 years.
Helios Delta is a reddish main sequence star of spectral class K2V and has a mass of 0.74. It was the last of the four systems to be settled by the Colonials. Helios Delta shares its orbit with Helios Gamma at a distance of 70 SU, and completes an orbit once every 456 years.
This system is unique in the fact that Aerilon, Hestia, and Canceron share the same orbit. The two colonies are positioned at Hestia's Lagrangian points, meaning that the gravitational pull of Helios Delta and Hestia are balanced, allowing two smaller bodies (Aerilon and Canceron) to share the same orbit in a "fixed" position, maintaining the same distance apart at all times. Aerilon sits at the (leading) L4 point, and Canceron at the (trailing) L5 point.
About 2,000 years prior to the Fall of the Twelve Colonies, the last twelve tribes of Kobol left their planet over conflicts with their gods, as well as some sort of calamity (TRS: "Kobol's Last Gleaming, Part I").
The tribes settled on twelve worlds some distance away (TRS: "Kobol's Last Gleaming, Part I"). The tribes' namesakes and icons originally corresponded to the twelve signs of the ancient tribes, although these names drifted over time (TRS: "Home, Part II").
The early Colonies lived (and fought) more as sovereign nations. Some (particularly Caprica) prospered, while others (such as Sagittaron and Aerilon) were often considered lessers. For labor forces as well as wars fought among each other, humanity created the Cylons. When these early models rebelled, the Colonies unified their governments under the Articles of Colonization at some point during the Cylon War into a federal republic known as the Twelve Colonies of Kobol.
40 years after the end of the devastating war, the Cylons return and sow destruction among the worlds of their creators (TRS: Miniseries, "The Plan"). For some time the Twelve Colonies remain occupied by Cylon forces and a small group of human survivors. After nine months, humanity and the Cylons leave the Cyrannus system forever (TRS: "Lay Down Your Burdens, Part II").
Out of all Colonial inhabitants, only about 55,000 survive the first days of the holocaust. Most of them leave the Colonies behind in two small fugitive fleets. The fleet of the battlestar Galactica eventually encounters both Kobol, as well as the original Thirteenth Colony Earth. Some time after that, and after the destruction of the the Cylon home world, they settle on a beautiful, rich world, leaving behind the baggage of their Colonial heritage. (TRS: Miniseries, "Razor", "Daybreak, Part II")
The ultimate fate of the Twelve Colonies 150,000 years after The Fall is unknown. Presumably, after several hundred years, their climates and surfaces would degrade until they resemble the original Earth.
It can be presumed that by present-day radiation levels would have lowered to a point where life would be allowed to flourish once again. But few, if any, traces would remain of the human civilizations that once thrived upon them, excluding remains in space, such as the remnants of Scorpion Fleet Shipyards or Ragnar Anchorage.
Ancient Name: Aries
Alternative Name: Aerelon
Population: 1.2 billion
Patron God: Demeter
Major Cities: Gaoth, Promethea
Attractions: University of Aerilon, Promethea A & M
Pyramid Teams: Aerilon Threshers, Promethea Golden Horns
About all of the planet's landmasses are situated in the temperate zones, featuring open and rolling landscapes offering little variation in altitude. There are few inland waterways, and the shorelines consist mostly of rocky bluffs with only mangrove swamps breaking the monotony near the equator. To top it the planet offers hostile seas with no useful harbors at its coasts.
Aerilon has a mild climate and reliable seasons, with plentiful rain, so catch basins and wells are sufficiently filled and fresh water is readily available despite the lack of rivers and lakes.
Ancient Name: Aquarius
Alternative Name: Aquarion
Patron God: Hermes
Major Cities: Heim
Attractions: Kyros Summerfest
Pyramid Teams: None
The planet surface is about entirely covered by vast, frigid oceans, punctuated with violently erupting volcanoes breaking through the sea surface. Only one land mass, named Kyros, is large enough to sustain human habitation. Volcanoes occupy each end of this crescent-shaped continent, which consist mostly of volcanic tephra covered in moss and dotted by exotic native plantlife.
Most people live in Heim, a town at the centre of the island facing what would be the harbor if the seas weren't so frigid. Only a few people live outside the town, tending the infrastructure needed for survival, like windmills, hydroponics, and solar panels. Due to the lack of wood, most structures are made of tin or stone.
Ancient Name: Cancer
Population: 6.7 billion
Patron God: Hephaestus
Major Cities: Hades, Prommos, Mangala
Attractions: Prommos, Ruby Range, Kor Yaz Glacier
Pyramid Teams: Canceron Hydras, Hades Vice, Mangala Krill
Prior to the First Cylon War, Canceron was a very overpopulated and poor colony. Canceron's major cities such as Hades and Mangala were known to have large slums. Its large population earned Canceron the nickname "The Largest Democracy" within the colonies, though some believed the government was nothing more than a corrupt Oligarchy. 
During the Cylon attack, Canceron is hit with at least three nuclear devices. One of the passenger liners within the fleet, Pyxis, is one of the last ships that escapes this world when the attack began. Aboard is the Number Six copy Shelly Godfrey (TRS: "The Plan").
Ancient Name: Capricorn
Population: 4.9 billion
Patron God: Apollo
Official Language: Caprican
Capital: Caprica City
Major Cities: Caprica City, Delphi, Phoebus
Pyramid Teams: Caprica City Buccaneers, Delphi Legion, Phoebus Suns
Caprica was regarded as the center of Colonial civilization, even long before the First Cylon War. Graystone Industries was one of the Colonies' most influential technology developers and had a hand in creating the very first Cylons. Some 60 years before the Fall, Caprica was also plagued by religiously motivated terrorism (CAP: Pilot). After the war, the planet was once again the seat of politics, culture, art, science, learning and even the Colonial government. It was also one of the wealthiest colonies (TRS: "Epiphanies", "Dirty Hands").
During the Cylon attack, Caprica and it's surrounding airspace, played host to numerous skirmishes, one of which would eventually determine the outcome of the war. Caprica herself, comes under heavy attack from Cylon forces right from the start. With numerous basestars jumping directly into orbit, they are quickly able to gain the upper hand. At least four battlestars, including the Yashuman and Valkyrie, are quickly dispatched, and numerous targets, including the once proud Caprica City, are annihilated from orbit by nuclear weapons (TRS: Miniseries, "The Plan"). The attacks leave the civilian shipping lanes in the vicinity of Caprica/Gemenon shut down, but the civilian ships are relatively ignored in the opening moments of the attack, in favor of military targets. This gives them time to gather and flee the area, picking up other survivors along the way. Eventually the fleet amasses over 100 ships of all kinds, both civilian and homeless military support craft. However, only 75 FTL capable ships are able to flee Caprican airspace, leaving the sub-lights, and their passengers, at the hands of the Cylons.
At the time of the attack, the Galactica was positioned 3 hours away, and was not able to partake in the defense of its home planet. It wasn't long, however, before the Cylons located its position, and attempted to take it out. After being struck by a nuclear warhead, suffering major damage, and not having anything to shoot with, but still in one piece, the Galactica had no choice but to retreat to the relative safety of the gas giant, Ragnar (TRS: Miniseries), leaving Caprica, and the remaining survivors, completely defenseless and on their own.
After the space-faring Colonials flee, Caprica is occupied by Cylon forces, attempting to undo the damage they've done, and colonize the planet themselves. They often clash with human survivors within the Colonies, and fight on this world for several weeks. The occupation and resistance both end roughly nine months after the attack and the planet is left for good (TRS: "33", "Resistance", "Lay Down Your Burdens, Part II", "The Plan").
6,250 people from Caprica join the Laura Roslin faction over Kobol (TRS: "Home, Part I"). The overall Caprican survivor population, possibly significantly greater, is unknown (see analysis here). An attempted settlement by the survivors of the holocaust is named New Caprica in honor of the original colony (TRS: "Lay Down Your Burdens, Part II").
The first Quorum of Twelve delegate after the Fall is Gaius Baltar. Years later this spot is taken by Lee Adama, the last Quroum delegate from that colony before Delegate Adama establishes the Quorum of Ship's Captains in the wake of the mass-execution of the Quorum during Gaeta's Mutiny (TRS: "Islanded in a Stream of Stars").
Ancient Name: Gemini
Population: 2.8 billion
Patron God: Hera
Official Language: Old Gemenese
Major Cities: Oranu, Illumini
Attractions: Kobol Colleges, Illumini Pantheon, Gramada Mountains, Spatiu Gol Plateau, Pustiu Desert
Pyramid Teams: Gemenon Twins, Illumini Vipers
Gemenon was the first world colonized by the humans of Kobol. It shares its orbit around Helios Alpha with Caprica, completing an rotation once every 28.2 days. Its status as a twin planet gives it its name. Its climate can vary wildly, featuring frozen plateaus, and searing deserts, which can make life difficult for its inhabitants.
Before the First Cylon War, Gemenon was home to the Kobol Colleges and a monotheistic cult called "The Soldiers of the One" (TRS: Miniseries, Night 2; CAP: Pilot). Gemenon's second-largest city, Illumini, was built around a large Pantheon complex, composed of buildings intended to worship and celebrate every deity in the Sacred Scrolls.
The Gemenese were known for their literal interpretations of the Scrolls (TRS: "Fragged"). Most of the population of Gemenon was apparently very strongly opposed to the federal laws legalizing abortion (TRS: "The Captain's Hand").
During the Cylon attack, Gemenon comes under heavy assault by Cylon air and ground forces, completely obliterating one city with a high-yield nuclear device, and causing extensive damage to another with conventional means. At least one ship, the Cybele, is able to escape the destruction of its port and occupying Cylon forces, and find Roslin's fleet (TRS: "The Plan").
Three months after the attacks, approximately 9,500 Gemenese break away from the main fleet, and join the Laura Roslin faction over Kobol (TRS: "Home, Part I"). The large and devoted Gemenese population continues to be an important political factor within the fugitive fleet (TRS: "The Captain's Hand").
Ancient Name: Leo
Population: 2.6 billion
Patron God: Artemis
Official Language: Leonese
Major Cities: Luminere, Hedon
Attractions: Royal Palace of Leonis, Hedon Grand Casino
Pyramid Teams: Leonis Wildcats, Hedon Suns
Leonis is the second planet in the Helios Beta system, and was geographically known for its open plains (TRS: "The Plan"). The planet itself has no axial tilt, which supports a mild and predictable climate. It was a major hub for both education and industry, and despite Colonial unification, was able to maintain its own language and culture.
In the era prior to the First Cylon War, Leonis produced and exported a popular energy drink named "Leonis Red". Leonis Estates Sparkling Wine was a type of alcohol produced on this colony (TRS: "Flight of the Phoenix").
During the final Cylon attack, Leonis is struck with at least one, low-yield nuclear device, that was able to stun its residents and disable any means of escape. Those who survived the blast, are then seen being systematically eliminated by both ground and air forces (TRS: "The Plan"). It is unknown if any are collected and used in the Four's various farms.
Ancient Name: Libra
Alternative Name: Libris
Population: 2.1 million
Patron God: Athena
Major Cities: Themis
Attractions: Supreme Hall of Justice, Themis Arena
Pyramid Teams: None
Ancient Name: Pisces
Population: 1.4 billion
Patron God: Poseidon
Major Cities: Perkinston, Queenstown, Pailyn
Attractions: Colonial Fleet Headquarters, Penrose Harbor
Pyramid Teams: Picon Panthers
After the Unification of the Twelve Colonies this world was the location of the Colonial Fleet Headquarters (TRS: Miniseries). Picon was also the home of Picon Laboratories, located in Pailyn, Muritolan (TRS: "Faith").
The colony came under heavy attack during the early stages of the Cylon attack. One of the last ships to leave the planet was the Rising Star, carrying many wounded people including an unconscious Ellen Tigh (TRS: "The Plan").
The destruction of Picon's Fleet Headquarters caused President Richard Adar to offer a complete and unconditional surrender to the Cylons. This offer was ignored by the invading forces (TRS: Miniseries).
Ancient Name: Sagittarius
Alternative Name: Sagittarion
Population: 1.7 billion
Patron God: Zeus
Attractions: Acheron River
Pyramid Teams: Sagittaron Archers
The Sagittarons were traditionalists who practiced a different form of religion from the other Colonials. They distrusted modern medicine and were generally unsupportive of the military (TRS: "The Woman King").
The Planet endured centuries of exploitation at the hands of the other Colonies. Eventually, Tom Zarek led an organized series of terrorist acts against the established government there, many years before the Fall of the Colonies (TRS: "Bastille Day").
Ancient Name: Scorpio
Population: 450 million
Patron God: Dionysus
Major Cities: Celeste, Argentum
Attractions: Argentum Bay, Guardian Mountains, Scorpia Shipyards
Pyramid Teams: Scorpia Stingers, Celeste Storms, Celeste Lightning, Argentum Bay Silverstars
Before the First Cylon War one of the major leisure spots of the world was Argentum Bay, despite the colony's trashy reputation.. The Planet was also apparently well-known for its Paragliding (TRS: "Razor").
After the Unification, Scorpia was home to the Scorpion Fleet Shipyards.
During the initial Cylon attack, Scorpia falls victim to numerous massive nuclear attacks, destroying much of the population. Cylon forces also focus on the orbiting ship yards, obliterating the structure, destroying numerous Battlestars and support ships, and causing another one to risk a blind jump in order to escape (TRS: "Razor").
Ancient Name: Taurus
Population: 2.5 billion
Patron God: Ares
Official Language: Tauron
Major Cities: Hypatia, Tauron City (Minos)
Attractions: Minos, Olympia Hippadrome, Great Tauron Plains
Pyramid Teams: Olympia Stallions, Tauron Bulls
Nearly a century before its Fall, Tauron was one of the poorer colonial worlds and the center of a civil war. A few years before the First Cylon War, the crime syndicate Ha'la'tha had enormous influence over the planet, which even extended to other colonies, such as Caprica (CAP: Pilot).
After the War, Tauron was known to be a somewhat troublesome colony within the federal system of the Colonies, often disobeying directives decided by the colonies and pushing their luck with the admiralty (TRS: "Hero"). Yet this world was counted among the wealthier of the twelve worlds (TRS: "Dirty Hands").
Ancient Name: Virgo
Population: 4.3 billion
Patron God: Hestia
Major Cities: Boskirk, Blaustad, Hadrian
Attractions: House of Parliament, Tower of Virgon, Petrus Palace, Virgon Universities
Pyramid Teams: Virgon United, Boskirk All Reds
Virgon had a declining monarchy and a strong national parliament, at least 58 years before the Fall of the Colonies.
Soon after the beginning of the Cylon attack, the Colonials launch a counter-attack over Virgon. The battle ends with the destruction of the flagship battlestar Atlantia and the resulting death of Fleet Admiral Nagala (TRS: Miniseries).
Marshall Bagot is the Virgon delegate to the first Quorum of Twelve. He nominates Tom Zarek for the vice-presidency (TRS: "Colonial Day"). In public ceremonies, the Virgon delegate wears a light blue sash.