Before expanding the Great Crusade to the stars beyond the Sol Sector in the late 30th Millennium, the Emperor of Mankind first ordered the construction of the Astronomican on Terra. The purpose of this colossal endeavour was that the entire device was to serve as a focus through which the Emperor could direct his fathomless psychic energies to generate a partly self-sustaining navigational beacon for the commerical and military starships of the Imperium. The psychic navigational beam generated by the Astronomican was able to cut through the Warp across the galaxy and those attuned to its frequencies and modulations, the sanctioned human mutants known as Navigators, were able to use it as a beacon and pole star when plotting journeys through the Immaterium.