The Aztec Calendar Stone was carved from solidified lava in the late 15th century. It somehow got lost for 300 years and was found in 1790, buried under the zocalo, or central square of Mexico City.
The stone — also called the Sun Stone or the Cuauhxicalli Eagle Bowl — went on display in the city’s Metropolitan Cathedral. About a century later, in 1885, it was moved to Mexico’s National Museum of Anthropology, where it remains to this day.
Weighing about 24 tons, the stone is around three feet thick and measures almost 12 feet across. We don’t know just how it was used — sacrificial altar? Sundial? Calendar? — but we do know that it predicts solar eclipses.
Its design is rich with animals and other symbols, all part of a complex cosmology. For example, the deity Tonatuih sits in the middle, holding a human heart in each hand. His outstretched tongue is a blade for ritual sacrifice, a common Aztec practice.
The stone depicts not one but two calendar systems, each totally different yet interlinked. Each day has two identities, one per system. Here’s a brief explanation of how they work.
– The 365-day xiuhpohualli describes the days and rituals by seasons, so it’s an agricultural year. It contains 18 months of 20 days each, plus five “unlucky” days when disasters were likely to happen. Circling the Sun Stone’s center there’s a band of the 20 symbols assigned to days.
– The 260-day tonalpohualli describes each day in terms of the Aztec gods, so it’s a sacred year. Every 24-hour cycle has one of 20 day names (such as jaguar, reed, wind, etc.) and a number from 1 to 13, because their weeks were 13 days long.
The way the two calendars interact, no date could be repeated for 18,980 days … or 52 years. The Aztecs believed that when the solar and sacred cycles fell on the same day, the universe was in great danger. So every 52 years, they performed an elaborate ceremony of human sacrifice and fire to ensure everyone’s survival. Well, everyone except the sacrificial victims.