Aztec calendar stone. 18th-century illustration of the Aztec calendar stone (Sun Stone, or Stone of the Five Eras) that was discovered in 1790 during excavations in the main square of Mexico City, Mexico. This 16th-century object is 3.5 metres across and nearly a metre thick. There are several interpretations of the carved symbols, which record Mexica (Aztec) cosmogony and chronology. The deity at centre may be the Sun god, surrounded by four previous eras (boxes) and concentric circles of date glyphs. This illustration is from the 1790 work by Mexican astronomer Antonio de Leon y Gama (1735-1802) describing the discovery of the calendar stone.