The Codex Borgia, a masterpiece that predates the Spanish conquest of central Mexico, records almanacs used in divination and astronomy. Within its beautifully painted screenfold pages is a section (pages 29-46) that shows a sequence of enigmatic pictures that have been the subject of debate for more than a century. Bringing insights from ethnohistory, anthropology, art history, and archaeo-astronomy to bear on this passage, Susan Milbrath presents a convincing new interpretation of Borgia 29-46 as a narrative of noteworthy astronomical events that occurred over the course of the year AD 1495-1496, set in the context of the central Mexican festival calendar. In contrast to scholars who have interpreted Borgia 29-46 as a mythic history of the heavens and the earth, Milbrath demonstrates that the narrative documents ancient Mesoamericans' understanding of real-time astronomy and natural history. Interpreting the screenfold's complex symbols in light of known astronomical events, she finds that Borgia 29-46 records such phenomena as a total solar eclipse in August 1496, a November meteor shower, a comet first sighted in February 1496, and the changing phases of Venus and Mercury. She also shows how the narrative is organized according to the eighteen-month festival calendar and how seasonal cycles in nature are represented in its imagery. This new understanding of the content and purpose of the Codex Borgia reveals this long-misunderstood narrative as the most important historical record of central Mexican astronomy on the eve of the Spanish conquest.