Rising solar seen from the stone causeway of the photo voltaic observatory on Mount Tlaloc, Mexico. The view aligns with the rising solar on February 24, coinciding with the Mexica calendar’s new 12 months. Credit: Ben Meissner
Aztec farming calendar precisely tracked seasons and leap years.
Without clocks or trendy instruments, historical Mexicans watched the solar to keep up a farming calendar that exactly tracked seasons and even adjusted for leap years.
Before the Spanish arrival in 1519, the Basin of Mexico’s agricultural system fed a inhabitants that was terribly massive for the time. Whereas Seville, the most important city middle in Spain, had a inhabitants of fewer than 50,000, the Basin, now often called Mexico City, was dwelling to as many as three million folks.
To feed so many individuals in a area with a dry spring and summer time monsoons required superior understanding of when differences due to the season in climate would arrive. Planting too early, or too late, might have proved disastrous. The failure of any calendar to regulate for leap-year fluctuations might even have led to crop failure.
Though colonial chroniclers documented using a calendar, it was not beforehand understood how the Mexica, or Aztecs, have been capable of obtain such accuracy. New University of California, Riverside (UCR) research demonstrates how they did it. They used the mountains of the Basin as a solar observatory, keeping track of the sunrise against the peaks of the Sierra Nevada mountains.
Solar observatory in Mount Tlaloc, Mexico. The inclined causeway aligns with the rising sun on February 23–24, in coincidence with the Mexica calendar’s new year. Credit: Ben Meissner
“We concluded they must have stood at a single spot, looking eastwards from one day to another, to tell the time of year by watching the rising sun,” said Exequiel Ezcurra, distinguished UCR professor of ecology who led the research.
To find that spot, the researchers studied Mexica manuscripts. These ancient texts referred to Mount Tlaloc, which lies east of the Basin. The research team explored the high mountains around the Basin and a temple at the mountain’s summit. Using astronomical computer models, they confirmed that a long causeway structure at the temple aligns with the rising sun on Feb. 24, the first day of the Aztec new year.
“Our hypothesis is that they used the whole Valley of Mexico. Their working instrument was the Basin itself. When the sun rose at a landmark point behind the Sierras, they knew it was time to start planting,” Ezcurra said.
The sun, as viewed from a fixed point on Earth, does not follow the same trajectory every day. In winter, it runs south of the celestial equator and rises toward the southeast. As summer approaches, because of the Earth’s tilt, sunrise moves northeast, a phenomenon called solar declination.
The stone causeway of the solar observatory in Mount Tlaloc, Mexico, aligns with the rising sun on February 23–24, in coincidence with Mexica calendar’s new year. Credit: Ben Meissner
This study may be the first to demonstrate how the Mexica were able to keep time using this principle, the sun, and the mountains as guiding landmarks. Though some may be familiar with the “Aztec calendar,” that is an incorrect name given to the Sun Stone, arguably the most famous work of Aztec sculpture used solely for ritual and ceremonial purposes.
“It did not have any practical use as a celestial observatory. Think of it as a monument, like Nelson’s Column in Trafalgar Square or Lincoln’s Memorial in Washington, D.C.,” Ezcurra said.
Learning about Aztec tools that did have practical use offers a lesson about the importance of using a variety of methods to solve questions about the natural world.
“The same goals can be achieved in different ways. It can be difficult to see that sometimes. We don’t always need to rely solely on modern technology,” Ezcurra said. “The Aztecs were just as good or better as the Europeans at keeping time, using their own methods.”
The Aztec observatory could also have a more modern function, according to Ezcurra.
Comparing old images of the Basin of Mexico to current ones shows how the forest is slowly climbing up Mount Tlaloc, likely as a result of an increase in average temperatures at lower elevation.
“In the 1940s the tree line was way below the summit. Now there are trees growing in the summit itself,” Ezcurra said. “What was an observatory for the ancients could also be an observatory for the 21st century, to understand global climate changes.”
Reference: “Ancient inhabitants of the Basin of Mexico kept an accurate agricultural calendar using sunrise observatories and mountain alignments” by Exequiel Ezcurra, Paula Ezcurra and Ben Meissner, 12 December 2022, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2215615119
