Precisas observaciones solares alimentaron a millones de personas en el México antiguo

El sol naciente se ve desde el corredor de piedra del Observatorio Solar en el Monte Tlaloc, México. El espectáculo coincide con la salida del sol del 24 de febrero, que coincide con el año nuevo en el calendario mexicano. Crédito: Ben Meisner

El calendario agrícola azteca seguía con precisión las estaciones y los años bisiestos.

Sin relojes ni herramientas modernas, los antiguos mexicanos seguían al sol para llevar un calendario agrícola que rastreaba con precisión las estaciones e incluso se ajustaba a los años bisiestos.

Antes de la llegada de los españoles en 1519, la Cuenca de México alimentaba a una población inusualmente grande para la época. Mientras que Sevilla, el centro urbano más grande de España, tenía una población de menos de 50.000 habitantes, la cuenca ahora conocida como Ciudad de México albergaba hasta 3 millones de personas.

Alimentar a tantas personas en una región con monzones secos de primavera y verano requería una mejor comprensión de cuándo ocurrirían las variaciones climáticas estacionales. Plantar demasiado temprano o demasiado tarde podría ser desastroso. El hecho de que cualquier calendario no se ajuste a las variaciones de los años bisiestos también podría resultar en una mala cosecha.

Aunque los cronistas coloniales documentaron el uso del calendario, antes no estaba claro cómo los mexicanos o los aztecas pudieron lograrlo.[{» attribute=»»>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