The New York Times reports:

Long before Europeans colonized North America, the Indigenous peoples in the valley where Mexico City would later arise may have followed a natural solar calendar that was so accurate it accounted for leap years.

The “horizon calendar,” proposed in a new study, relied on natural landmarks in the valley’s rugged eastern mountains, and was kept in sync with the astronomical year by a temple atop a sacred volcano. The system may have been used by the Aztec culture, which flourished in the area from roughly 1300 to 1500, though previous civilizations also referred to the horizon to tell time.

Earth’s trip around the sun isn’t divided into 365 perfectly discrete days. Every year, there is an extra quarter of a day. Failing to account for that time can add up, disrupting a civilization’s calendar. Tracking the days was a concern in the valley, known as the Basin of Mexico, where as many as three million people lived before Europeans arrived, making it among the most populated regions on the planet at the time. With so many mouths to feed, corn farmers had to time their planting precisely.

While the Mesoamerican peoples lacked compasses, quadrants or astrolabes, their method for keeping their agricultural calendar may have been hiding in plain sight around the basin, said Exequiel Ezcurra, a distinguished professor of ecology at the University of California, Riverside, and an author of the study, which was published on Monday in The Proceedings of the National Academy of Sciences. [Continue reading…]