Earth is the home of the lost Thirteenth Tribe in both Battlestar Galactica sagas.
In the Original Series
The lost Thirteenth Tribe settles on the planet Earth, its settlers having departed long ago from Kobol after the same environmental cataclysm which caused the Exodus of the Twelve Tribes and made Kobol uninhabitable. As stated in the Book of the Word, the Thirteenth Tribe left opposite the other Twelve Tribes, the information of their destination residing in the tomb of the Ninth Lord of Kobol. (Lost Planet of the Gods)
Transmissions from Earth -- i.e. the Apollo Moon Landing -- were picked up by Captain Apollo in the Celestial chamber on an obsolete Gamma frequency, though the copy of the transmission was destroyed in the attack on the Cylon basestar. (The Hand of God)
Earth is Found
In Galactica 1980, Earth was discovered, but, much to the dismay of Adama and the survivors, the humans on Earth were underdeveloped and would not be able to defend themselves from the Cylons.
Many fans of the Original Series feel that the events of "Galactica 1980" are apocryphal, dispite the airing of this information, preferring to consider Earth as undiscovered as of the conclusion of the Original Series. (Regardless of its popularity, Battlestar Wiki maintains its canonicity for purposes of this encyclopedia.)
Notes
Technology from Earth appears to be present at Athena's station in "Saga of a Star World." As she runs a diagnostic on Starbuck's Viper, the words "MADE IN USA" appear on the screen. This is obviously a cinematic goof, as the Colonials knew very little of Earth, much less the countries that populated it.
In the Re-imagined Series
Earth is the alleged location of the Thirteenth Colony, destination of the Thirteenth Tribe of Kobol, as attested in the Sacred Scrolls. Earth is first mentioned by Commander Adama during a speech made after funeral services in the wake of the devastating Cylon attack on the Twelve Colonies, claiming that it is a real place and that top military leadership knew its location (see Miniseries). In a private conversation with new President Laura Roslin immediately afterward, Adama admits that he has not the slightest idea where Earth may be, if it exists. His true motivation was inspirational; he intended to bolster the morale of the population (especially his beleaguered crew) in the aftermath of the near-annihilation of humanity.
While she is initially skeptical, going so far as to rebuke Adama for his dishonesty, Roslin's faith grows perceptibly throughout the events of the first season. This transition is at least partly due to her use of the hallucinogen Chamalla as an alternative treatment to her terminal breast cancer. These hallucinations are increasingly interpreted as prophetic vision by Roslin, and they continue as the Fleet ultimately finds the legendary Kobol in "Kobol's Last Gleaming, Part I."
Eventually, a religious artifact is recovered which later aids in revealing the general location of Earth (Home, Part II).
In a mysterious chamber, the stars of the Twelve Colonies are represented in the twelve Zodiacal constellations, as viewed from Earth, recognized by Roslin as the shapes of the original flags of the Twelve Colonies. The chamber itself was intended by its builders to be a stylized replica of the night sky of the Earth, from which all twelve Colonies could be seen. This is corroborated by Starbuck, who quotes scripture, saying that Earth was the place where the people of the Thirteenth Tribe could look up into the sky and see their twelve brothers. Captain Apollo also recognized the Lagoon Nebula, also represented in the night sky of the mystery chamber. This astronomical object is known to Fleet personnel, since Commander Adama remarked that this celestial body would take some time to reach.
It's presumed that some sort of recording of the chamber was taken to allow the fleet to match the position of stars to the view from an potiential candidate for Earth.
Is Earth the birthplace of Humanity in the re-imagined Battlestar Galactica?
Throughout the re-imagined series of Battlestar Galactica, there has been evidence (both circumstantial and non-circumstantial) that the humans of Kobol originated from the planet Earth and not Kobol itself. If humanity did originate from the planet Earth, then the Thirteenth Tribe of Kobol was returning to Earth, which could explain why the holographic star maps in the Tomb of Athena were made in a way to resemble the night sky on Earth as well as the location and name of the Lagoon Nebula.
The debate about the origins of humanity and which planet is the "real" birthplace of humanity still remains a highly contested issue among fans of Battlestar Galactica, especially the fans of the re-imagned series.
Notes
- In one of the extras included on the five-disc DVD release of the first season of Battlestar Galactica, Edward James Olmos (the actor portraying Commander Adama) remarks in a very jocular fashion that they will never reach Earth. Mary McDonnell (the actress who plays Laura Roslin) covers his mouth and tries to hush him while laughing. Although loyal fans may question the trustworthiness of this statement, it is known that the creators keep a "Bible" to the show, to which the actors presumably have access.