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Precise Solar Observations Fed Millions in Ancient Mexico
Aztec farming calendar accurately tracked seasons, leap years [ucr.edu]:
Without clocks or modern tools, ancient Mexicans watched the sun to maintain a farming calendar that precisely tracked seasons and even adjusted for leap years.
Before the Spanish arrival in 1519, the Basin of Mexico's agricultural system fed a population that was extraordinarily large for the time. Whereas Seville, the largest urban center in Spain, had a population of fewer than 50,000, the Basin, now known as Mexico City, was home to as many as 3 million people.
To feed so many people in a region with a dry spring and summer monsoons required advanced understanding of when seasonal variations in weather would arrive. Planting too early, or too late, could have proved disastrous. The failure of any calendar to adjust for leap-year fluctuations could also have led to crop failure.
Though colonial chroniclers documented the use of a calendar, it was not previously understood how the Mexica, or Aztecs, were able to achieve such accuracy. New UC Riverside research, published in the Proceedings of the National Academy of Sciences, 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.
"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.
Journal Reference:
Exequiel Ezcurra, Paula Ezcurra, and Ben Meissner, Ancient inhabitants of the Basin of Mexico kept an accurate agricultural calendar using sunrise observatories and mountain alignments [open], PNAS, 2022. DOI: https://doi.org/10.1073/pnas.2215615119 [doi.org]
