This calendar helped the Mexica, or Aztecs, to plant crops in a way to avoid hot dry springs and summer monsoons.
The Basin of Mexico holds Mexico City as well as the ruins of past Indigenous cities like Tenochtitlan and Teotihuacan, according to Discovery Magazine. Way back in 1519, the Basin of Mexico's agricultural system fed a massive population of as many as 3 million people. The Basin, now known as Mexico City, had to ensure planting of crops was done in a timely manner based on seasonal variations in weather. If they planted too early or late, it could end crop failure which would have been a disaster. So the Mexica, or Aztecs, came up with a sophisticated way of using the sun and mountains to maintain a farming calendar that precisely tracked seasons and even adjusted for leap years, according to Science Daily.
UC Riverside research in a paper published on December 12 in the journal Proceedings of the National Academy of Sciences reveals that they used the mountains of the Basin as a solar observatory. They would watch the sunrise against the peaks of the Sierra Nevada mountains to determine their findings. "A farming calendar that is not adjusted to the seasons eventually will have no purpose," said Exequiel Ezcurra, distinguished UCR professor of ecology who led the research. “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,” according to the researcher.
That single spot was found to be Mount Tlaloc, which lies east of the Basin. "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. Using computer modeling of the sunrise, the researchers determined that atop this sacred mountain lies a temple. The researchers identified the sun would have risen precisely behind a long causeway at the temple on Feb. 24, which marked the beginning of the Mexica new year. The Mexica noted the time with the mountains as reference points while using the fact that the sun, when viewed from a fixed point on Earth, does not follow the same trajectory every day. The sun runs south of the celestial equator and rises toward the southeast in winter. And in summer sunrise moves northeast, a phenomenon called solar declination because of the Earth's tilt.
On this day in 1790, the Aztec Sun Stone was re-discovered in Mexico City. Carved from a single piece of porphyry, the original weighs >20 tons! It doesn’t depict a calendar—it marks a crucial Aztec military victory. A replica is in the Museum’s Hall of Mexico & Central America. pic.twitter.com/fTOhZhDhFU— American Museum of Natural History (@AMNH) December 17, 2018
What's more interesting this shows that the time-keeping method used in ancient Mexico was not the Sun Stone as often believed. It is often incorrectly described as the "Aztec calendar". The Sun Stone was instead used solely for ritual and ceremonial purposes. "It did not have any practical use as a celestial observatory," Ezcurra said of the Sun Stone. "Think of it as a monument, like Nelson's Column in Trafalgar Square or Lincoln's Memorial in Washington, D.C." Ezcurra added, "The Aztecs were just as good or better as the Europeans at keeping time, using their own methods. The same goals can be achieved in different ways. It can be difficult to see that sometimes."