Exequiel Ezcurra is not an archaeologist. He is an ecologist and conservationist currently investigating the origin and evolution of maize farming in Mexico. In his research this time, he discovered that the mountain was an ancient cosmic observatory.
While exploring past agricultural techniques, he discovered something else: early inhabitants of the Basin of Mexico used ruins on a nearby mountain to keep accurate agricultural calendars.
Ezcurra published these findings with two other collaborators, his daughter and a friend who took drone footage in the scientific journal Proceedings of the National Academies of Sciences (PNAS).
Prior to the Spanish conquest, the peoples of the Mexico Basin had a very impressive agricultural system. The monsoon season comes in the summer, but the 'fake rain' at the start of the year can tempt farmers to plant their crops prematurely to their detriment.
But that didn't happen, and this shows how great their knowledge of the calendar is. As Ezcurra explained, this finding was very important, considering that local people at that time did not have any navigational and observational instruments that Europeans had at that time, such as compasses, astrolabe and quadrants.
"My initial motivation was to find out how this all started and how these people came up with very productive agricultural systems early in human history," he said as quoted by Vice.
He believes, the answer is the mountains in the region. Many other civilizations demonstrated 'firmament calendars' which recorded the position of the Sun rising (or setting) relative to geographical features along the horizon.
Ezcurra and colleagues began investigating the different horizontal landmarks, cross-referencing them with ancient codices to see if there was a reference. It was through this process that they decided to focus on Mount Tlaloc, a possible candidate due to its proximity to the east.
"If you want to use a horizon landmark, you want it to be as close to the east as possible because that's where the Sun turns as Sun rises and shifts markedly from one day to the next," explains Ezcurra.
Using astronomical models, the team found that one of the ruins on Mount Tlaloc, a long and perfectly straight stone causeway, aligns with the sunrise on February 23/24.
"If an observer were standing at the bottom of this stone causeway, looking up on February 23 or 24, the observer would see the Sun right in the center of the axis," Ezcurra said.
What is remarkable, he insists, is that February 23 is the date previously identified by historians as the new year marker in the Mexica calendar. The mountain, says Ezcurra, is a natural observatory aligned with the Sun rising on the one day of the year that coincides with the start of the Aztec new year.
Ezcurra hopes that other people, especially those with archaeological and astronomical backgrounds, will develop and further explore this discovery.
He also thought that the site itself should be preserved by archaeological and environmental authorities, as well as respected by Mexicans like himself.
"It is a pride that the early inhabitants were able to understand the seasons so precisely. The site must be protected," he concluded.