An Ancient ‘Horizon Calendar’ Comes Into View Over Mexico City

That result, he said, is “exciting” because it “means that they were not only using observation points from the bottom of the basin against the mountains, but they were able to construct an observatory on the summit of the mountain that had the same alignments that they were getting from the bottom.”

The study offers a tantalizing vision of terrain-based timekeeping, but not all researchers agree with its conclusions. Ivan Sprajc, a specialist in Mesoamerican archaeoastronomy at the Slovenian Academy of Sciences and Arts, pointed to earlier research indicating that these eastern mountains were used to identify specific dates, rendering the team’s findings “not novel at all” in his view.

He added that the existing body of evidence strongly suggested that Aztecs, and their predecessors, never developed a correction system that incorporated leap years.

“My general opinion of this study is that it does not rely on compelling evidence and simply ignores a large amount of previous systematic research on Mesoamerican architectural alignments,” Dr. Sprajc said.

Not all experts were so scathing. Anthony Aveni, professor emeritus of astronomy, anthropology and Native American studies at Colgate University in Hamilton, N.Y., called the new work “a solid, well-documented contribution to Mesoamerican astronomy.”

“The alignments toward features on the eastern horizon in the Valley of Mexico, also involving Cerro Tlaloc, are well documented during the Aztec period, and here we have good evidence that these sorts of practice were in effect much earlier,” Dr. Aveni said, who reviewed the study for the journal that published it.

Dr. Ezcurra, for his part, said he had expected pushback from experts, given his unconventional expertise for this topic. Yet he hopes the new study raises more awareness of the natural wonders and rich heritage in the Basin of Mexico, where skywatchers have looked to the rising sun for centuries to orient themselves in space and time.

The basin’s mountains are “telling us stories of cultural evolution in the past that have become lost,” Dr. Ezcurra said.

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