Aztec cosmology, as recorded in Sahagun's Florentine Codex and analyzed at length by Miguel Leon-Portilla, held that the present world was the fifth in a sequence of created and destroyed cosmic eras, each ended by a different catastrophe, and that its sun came into being at Teotihuacan when the gods gathered in darkness to determine who would sacrifice himself to become the new sun. The proud, wealthy god Tecciztecatl hesitated at the edge of the sacrificial fire, but the humble, disease-marked Nanahuatzin leapt in without flinching, and Tecciztecatl, shamed into following, became the moon, its light dimmed relative to the sun as punishment for his initial cowardice.
Even after this act, the newborn sun remained motionless at the horizon, refusing to cross the sky until the assembled gods sacrificed themselves as well, their hearts and blood setting the sun into its daily motion. This founding act established the theological premise underlying Nahui Ollin: motion and life themselves had cost the gods everything, and humanity, born into a world already purchased by divine self-sacrifice, inherited an ongoing obligation to repay that debt in kind or risk the sun's motion failing again.