Mostly harmless
In the Re-imagining[edit]
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 Mini-series). In a private conversation with newly-appointed President Laura Roslin, Adama admits that he has no clue 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, whose side-effects she increasingly interprets as prophetic vision. These visions start in the episode Flesh and Bone with a portentious dream in which she sees Leoben Conoy before he is discovered hiding in the fleet. She experiences a more disturbing vision in The Hand of God, when imaginary snakes covering her pulpit disturb her during a press conference on the Colonial One. During this same episode, she confides in Elosha the priestess of her previous vision concerning Corben.
Her visions continue as the Fleet actually finds the legendary Kobol in Kobol's Last Gleaming, Part I, leading ultimately to her temporary deposition. Eventually, the Arrow of Apollo is recovered from Caprica, the Tomb of Athena is opened, and clues to the location of Earth are discovered in 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. Cpt Apollo also visually recognized the Wikipedia:Lagoon Nebula, also represented in the night sky of the mystery chamber. Apparently this astronomical object is very well known to Fleet personnel, since Commander Adama remarked that this celestial body would take some time to reach.
As a side note, in one of the Extras included on the five-disc DVD release of the first season of Battlestar Galactica, Eddie 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), who just so happens to be there, 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.
In the Original Series[edit]
Earth is the Thirteenth Colony; 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 Temple on an obsolete Gamma Frequency, though the copy of the transmission was destroyed in the attack on the Cylon basestar. (The Hand of God)
Notes (Non-Canonical)[edit]
In the nefarious Galactica 1980, Earth was discovered. 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.