Aztec 'Reckoning of the Sun' cycle. 18th-century illustration of the 52-year Aztec cycle called 'Reckoning of the Sun'. The cycle of 52 years is subdivided into four equal parts of 13 years each, called Tlalpilli. The successive years are labelled with four signs: Calli (House); Tecpatl (Flint); Acatl (Reed); and Tochtli (Rabbit). These signs are illustrated pictorially and the number of years indicated by dots. An astronomical festival was held every 52 years to mark the passing of each cycle. This illustration is from a 1790 work by Mexican astronomer Antonio de Leon y Gama (1735-1802) describing the discovery of an Aztec calendar stone in Mexico City that year.