Modern and past indigenous peoples, anthropologists, and even centuries-old missionaries have long known about many of the ritual and symbolic aspects of the complex calendar system of the Aztecs, also known as the Mexica.
This calendar system involves an interweaving of a solar and ritual calendar; the normal solar calendar had 365 days, while the ritual calendar had 260 days and was divided into 13 groups of 20 days each. (Each of these days represented a unique combination of 13-day and 20-day associations and had its own name, symbol, patron deity and designation of good or evil.)
But how they calibrated their solar year has long been a mystery. In fact, until recently, scientists still haven’t deciphered why the Aztec New Year started when it did – February 23rd in the Gregorian calendar, which is used by Western cultures today.
“It was a conundrum for us,” says Ezekiel Escura, an ecologist at the University of California, Riverside and lead author of a study published in PNAS which aimed to find the answer.
Although an ecologist by profession, Escura has always been interested in the environmental history of the Basin of Mexico that holds Mexico City, as well as the ruins of past indigenous cities such as Tenochtitlan and Teotihuacan. The entire area is subject to seasonal monsoons that dictate the agricultural year. Given the huge populations the area has supported, past and present, Ezcurra says having accurate measurements of the solar year would be important for calibrating planting and harvesting seasons.
While the Aztecs knew that a solar year contained about 365 days, just like we do, they needed a way to recalibrate that calendar—since it actually takes the Earth 365 days, 5 hours, and 49 minutes to orbit the Sun. . In the modern Gregorian calendar, we get around this hard fact by adding an extra day every four years (known as a leap year), then skipping a leap year every 100 years for three out of every four centuries.
But simply missing a leap year in a 365-day calendar, for example, may have caused the Aztecs to miss the ideal planting season by a week after several centuries had passed. “A farming calendar that doesn’t match the seasons won’t make sense in the end,” Escura says.
Until this latest research, scientists had yet to find a specific calibration device used by the Aztecs. But Escura suspected that they used the horizon itself as a calendar tool, specifically using the day the sun first hit certain points in the distance while standing at a fixed point to keep track of the time.
Bird’s eye view
He and his colleagues began studying the Aztec codices — books the Aztecs wrote about themselves — and early Spanish colonial writings for clues. They also conducted astronomical simulations, eventually finding that a person stands on top of The mayor of Templo in Tenochtitlan (the capital of the Aztec Empire) could keep a pretty good clock on the calendar.
On March 16 in the Gregorian calendar, the sun would touch Mount Tlaloc for the first time, marking the beginning of the dry spring that precedes the rainy season. At the height of the corn growing season, when the Aztecs celebrated Chicomecoatl, goddess of corn, the sun rose behind the corn terraces for the first time. And on the winter solstice, a time when the Aztecs celebrated Ilama Tecuhtli, or Tona, the mother goddess, the sun rose over a snow-covered silhouette of the Iztaccíhuatl volcano, southeast of Tenochtitlan, which roughly translates as “The White Woman.”
“We were amazed to find,” Escura says, “that their rituals and celebrations match what you see on the far horizon from the temple.”
Mysterious New Year
Yet this amazing discovery still does not reveal anything clear about the Aztec New Year. When you’re standing near the Templo Mayor looking east on February 23, the sun isn’t rising anywhere near anything important.
But when we think about the history of the Basin of Mexico, the Aztecs were hardly the first culture around to need an accurate calendar to calibrate their planting and harvesting. Cultures and cities flourished there centuries before the people of Mexico founded Tenochtitlan in 1325 AD.
So instead of sticking to Aztec ruins and structures, Escura and his colleagues began looking at other, older sacred sites around the basin. Using more astronomical simulations, they found “something that blew our minds,” he says: The sun will rise directly over Mount Tlaloc when viewed from Tepeyac, the place where tradition says Our Lady of Guadalupe appeared to native Mexicans on February 23, 1531 AD.. The hill was a sacred place both during and before the rise of the Aztecs.
Intrigued, the researchers then looked at aerial images of Mount Tlaloc and found the ruins of a strange structure aligned with Mount Tepeyac. It consisted of two long walls which formed a sort of open-air corridor leading about 492 feet into a square yard at the rear.
On February 25, researchers visited the ruins. Although they arrived two days off after the Aztec New Year, they could still check the alignment of the corridor with a monolith in the sloping backyard and a circular stone marker at the entrance to the corridor. The sun was falling straight down the corridor, hitting the monolith and the stone marker.
Other researchers previously dated the structure to sometime between the collapse of Teotihuacan in the mid-seventh century AD and the rise of the Aztecs in the 14th century. Ceramic shards found around it reveal that several cultures used the area before the Mexica, although researchers haven’t been able to figure out what.
“People in the Basin of Mexico for many years before the arrival of the Aztecs used the alignment between Tepeyac and Mount Tlaloc to adjust their calendar,” says Escura.
He and his colleagues were not the only people on the hill that cheerful February morning; about a dozen other people also made the pilgrimage – some left gifts at the monolith where the sun hit, including food, wildflowers, chocolate and ceramic vases. Someone even leaned a skateboard next to the monolith, decorated with the painted image of Tlalocthe god of rain.
“They all paid tribute to Tlaloc with a donation,” Escura says. He talked to some, but none of them seemed to know specifically the significance of the day – one person told him that about a dozen people make a pilgrimage up the mountain every day. It is clear that the area still has some cultural significance for the people who live in the surrounding villages and suburbs of Mexico City.