If you’re a fan of avocado toast or guacamole, chances are the tasty green treat you’re eating was grown in Mexico. In 2019, the United States imported $28 billion worth of agricultural products from Mexico, with fresh fruits and vegetables leading the way.
It turns out that Mexican agricultural dominance dates back centuries, long before Spanish colonization began in 1519. Before the arrival of the Spanish, the agricultural system in the Basin of Mexico, a 3,700-square-mile highland plateau in central Mexico, fed a huge population for the time. Mexico City (called Tenochtitlan) was home to a whopping 3 million people, compared to 50,000 in Seville, Spain’s largest urban center.
A study published today in the journal Procedures of the National Academy of Sciences (PNAS) describes how the Mexica, or Aztecs, were able to create such an accurate agricultural calendar.
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An accurate calendar was crucial to growing the food that fed so many people in a region of dry spring and summer monsoons. Farmers needed an advanced understanding of when these seasonal variations in weather would kick in, as planting crops too early or too late could be disastrous. They also needed a calendar that could adjust to the leap year.
Colonial chroniclers documented the use of a calendar, but this new research shows that the Mexica used the basin’s mountains as a solar observatory, tracking the sunrise against the peaks of the Sierra Nevada mountains.
“We came to the conclusion that they must have been standing in a single spot, looking east overnight, reading the time of year by looking at the rising sun,” says Exequiel Ezcurra, the study’s lead author and professor of ecology at the University of California. Riverside, said in a statement.
To find the site, the team analyzed Mexica manuscripts, particularly those that referenced Mount Tlaloc. The mountain on the east side of the basin had a temple on top. Using astronomical computer models, the team confirmed that a long causeway-like structure near the temple aligns with the rising sun on Feb. 24. Depending on which calendar (Gregorian or Julian) is used as a comparison, February 23 or 24 is the first day of the Aztec New Year.
“Our hypothesis is that they used the entire valley of Mexico. Their working tool was the pelvis itself. When the sun came up on a landmark beyond the Sierras, they knew it was time to start planting,” Ezcurra added.
Seen from a fixed point on Earth, the sun does not follow the same path every day. During winter, the sun is south of the celestial equator and rises to the southeast. As the longer days of summer approach, the sunrise moves to the northeast due to the tilt of the Earth. This process is called solar declination.
This study may be the first to demonstrate how the Mexica were able to keep track of time using this principle with the sun and mountains as landmarks. Learning about these Aztec methods offers a lesson on the importance of using different techniques to solve questions about the natural world.
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“The Aztecs were as good or better than the Europeans at keeping time, with their own methods,” Ezcurra said.
The observatory could also have a modern function today. Historical footage shows the forest slowly ascending Mount Tlaloc, possibly due to an increase in average temperatures at lower elevations.
“In the 1940s, the tree line was well below the summit. Now trees are growing on the summit itself,” Ezcurra said. “What was an observatory for the ancients can also be an observatory for the 21st century, to understand global climate changes.”