The Aztecs apparently used a sophisticated combination of natural landmarks and structures to synchronize their calendar with the sun, archaeologists have found. For example, a paved stone path at the top of Mount Tlaloc was aligned with the precise point on the horizon where the sun rose on the Aztec New Year. Their solar calendar may have allowed the Aztecs to coordinate their agriculture with the rainy season.
The Aztecs ruled much of what is now Mexico until the arrival of the Spanish conquerors in 1519. At that time, one to three million people lived in the Mexican Basin alone, in which Mexico City is located today. This was an unusually high population density for the time – and a lot of people who wanted to be fed through agriculture. However, this was not easy in this monsoon-ridden region, as rain only fell at certain times of the year. Spring was typically dry and only in early summer did the rainy season ensure sufficient irrigation of the fields.
Aztec agriculture according to the calendar
For Aztec agriculture, this meant, “If they planted their corn when it first rained, they might have come too early because it was an isolated rainfall and not yet the beginning of the rainy season,” explains Exequiel Ezcurra of the University of California at Riverside and his colleagues. “On the other hand, if they planted too late, the plants didn’t have enough time until the end of the rainy season.” To avoid this, Aztec farmers had to have a schedule that allowed them to prepare their corn fields well in advance of the rainy season, and then apply the crops at the right time. In fact, historical documents from the time of the Conquista show that the Aztecs had a calendar in which the days for certain field work were also firmly defined.
In order for such a calendar to stay in sync with the seasons and the course of the sun, the members of this pre-Columbian culture had to be able to adjust it again and again with the help of astronomical timers – for example by tracking key dates of the solar year such as the solstices or certain sunrise positions of the sun. In fact, many Aztec temples were aligned with such astronomical markers. “This architectural orientation had great symbolic, ritual and cultural significance, but would not have been accurate enough for the calendar comparison,” the archaeologists explain. But how the residents of Tenochtitlan managed this was previously unclear.
Mountain peaks and temple path as calendar marks
Now Ezcurra and his team have discovered how the Aztecs in the Mexican Basin synchronized their calendar with the sun. The first clues came from historical texts in which Mount Tlaloc, to the east of the depression, played a prominent role. There is also an Aztec temple complex on its summit. Using surveys, surveys and a computer model, the archaeologists found that the sun appears to rise directly behind the peak of Tlaloc only two days a year – once in spring and once in autumn. “The number of days in a year can therefore be determined by counting from the day the sun rises behind Tlaloc in spring to the next spring sunrise,” the researchers said.
The scientists found further indications of a calendar function of this mountain on Mount Tlaloc: In the temple complex there there is a 150 meter long, six meter wide stone path that runs dead straight up the mountain. “Looking uphill along this path, the point at which it meets the horizon defines exactly the position where the sun will fall on December 23/24 each year. February seems to be rising,” report Ezcurra and his team. This day marked the beginning of a new year for the Aztecs. “The idea that this stone path was used for precise astronomical observations is supported by the fact that there were specific sighting marks there,” where the archaeologists continue. A stone circle at the top and a stone block with a 40 centimeter high monolith on it at the bottom of the stone path may have helped the Aztecs to aim for the rising sun.
“These results confirm that the people of the Mexican Basin had an extremely precise calendar even before the arrival of the Europeans and their astronomical tools,” the archaeologists state. “By systematically observing the sunrise behind the eastern mountain range of the basin, they were able to synchronize this calendar and even plan for leap years.”
Source: University of California Riverside; Specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2215615119