Aztec calendar divisions with the Moon. 18th-century illustration of the division of an Aztec year ('xiuhpohualli') into 18 equal non-lunar periods of 20 days called veintena. Anti-clockwise from top: Atlcahualo, Tlacaxipehualiztli, Tozoztontli, Huey Tozoztli, Toxcatl, Etzalcualiztli, Tecuilhuitontli, Huey Tecuilhuitl, Miccailhuitontli, Huey Miccailhuitontli, Ochpaniztli, Teotleco, Tepeilhuitl, Quecholli, Panquetzaliztli, Atemoztli, Tititl, Izcalli. An extra 5-day nemontemi period made 365 days in total. 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.