Aztecs kept accurate time using the sun and the mountains

When the Spanish arrived in 1519, the Basin of Mexico supported an unusually large population of indigenous Aztec people. Whereas Seville, the largest urban center in Spain, had a population of fewer than 50,000 residents, the Basin, now known as Mexico City, was home to as many as 3 million. Food for this population was cultivated using an extraordinarily precise agricultural system that relied on an accurate calendar to identify when to till the land, plant the crops and take in the harvest. 

This region has a dry spring, with summer monsoons, making it critical not to plant the maize crops too early or too late. If seeds were planted too early, in response to a random early rainfall event, all the young seedlings could die before the true monsoon weather arrived. If seeds were planted too late, the plants would have a short growing season and end up producing a lower yield. Considering that the Aztecs of the time did not have the celestial instruments used by Europeans (e.g., gnomon, compass, quadrant, and astrolabe), scientists have always wondered how they knew exactly when the day for planting (or harvesting) arrived. 

New research by scientists from the University of California Riverside, and colleagues, has now combined information from the records of colonial chroniclers and from detailed measurements of coordinates referencing the mountains around the Basin of Mexico and the movement of the sun. With this information, the researchers set out to understand whether the Aztecs perhaps used the mountains of the Basin as a solar observatory, and kept track of the days by monitoring the position of sunrise against the peaks of these mountains.

Many scholars believe that the Aztec calendar-keepers used the sun’s position at sunrise against the prominent peaks on the basin’s mountainous horizon as calendrical landmarks, a “horizon calendar” that provided accurate indicators of specific dates along the solar year. The use of the landscape as a calendric tool is based on the fact that, because of the tilting of the Earth, the point on the horizon where the sun rises shifts from day-to-day throughout the year.

As a result of their initial investigations, the researchers hypothesized that the Aztecs used the eastern mountains of the Basin of Mexico to adjust the agricultural calendar to the solar year. They were also able to use this very precise system to recognize leap years and add in the extra day that keeps the calendar year synchronized with the solar year.  “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 built at the temple aligns exactly with the rising sun on February 24 each year; this is known to be 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.

According to the researchers, this landmark point was most likely to have been Mount Tlaloc. The Aztecs could have counted the number of days in a year, starting from the day on which the sun rises behind Mount Tlaloc in spring, and ending on the next springtime sunrise behind Mt Tlaloc. This would give 365 days. In the Aztec system, there were 18 “months” in a year, each one consisting of 20 days. This was followed by 5 nemontemi, or “useless” days, at the end of the year. After four years of counting the days in this way, a calendar-keeper would have had to add an extra day to the year, in order to keep the sun rising behind the peak of Mount Tlaloc on the same day of the year. 

Landscape features are still often used to mark calendric dates in many traditional villages. Despite modern communications and the standardization of calendric time, many communities still have ceremonies related to planting or harvest that are celebrated when sunrise or sunset is aligned with landscape features that act as reference points. For example, the sun rises over the Heel Stone at Stonehenge, in the UK, each year on the 21st June, marking the summer solstice and the longest day of the year.  

This study may be the first to demonstrate how the Mexica were able to keep time using the effect of Earth’s tilt, the sun, and the mountains as guiding landmarks. In addition, it illustrates how humans can use a variety of methods to solve questions they have 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 as, or better than, 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 on 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.”

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By Alison Bosman, Earth.com Staff Writer

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