Mesoamericans Have Been Using a 260-Day Ceremonial Calendar for Millennia

In parts of Mesoamerica today, Indigenous communities use a 260-day ceremonial calendar. Now, fresh evidence suggests the practice dates back at least 3,000 years. Newly uncovered ruins along Mexico’s southern Gulf Coast appear to have been designed in alignment with the ancient timekeeping system.

Aerial surveys using lidar technology revealed that hundreds of architectural complexes were aligned to facilitate timed observations of the rising and setting sun, moon and other celestial objects in line with this 260-day cycle. Scientists had suspected that the calendar, which is tied to cycles of maize agriculture and human reproduction, dated back this far. But the earliest documented evidence for its use was a glyph depicting “7 Deer,” one of the days in the calendar, as part of a third-century B.C.E. mural in Guatemala. Since these cultures didn’t leave written records from earlier periods, scientists have found it exceedingly difficult to establish proof of prior calendar use—until this new large-scale discovery.

The researchers behind the findings published their results in Science Advances on Friday.

These monumental assemblages of plazas, pyramids and platforms, some stretching more than half a mile, indicate the 260-day cycle was likely of central importance to the Olmec, Maya and other cultures since at least the key period of time around 1000 B.C.E.—when more widespread maize agriculture began to take hold in the region.

“It is obvious that the orientations reflect a complex worldview in which astronomical knowledge conditioned by practical concerns was intertwined with religious concepts,” says co-author Ivan Šprajc, who studies Mesoamerican archaeology and archaeoastronomy at the Slovenian Academy of Sciences and Arts.

For centuries, many of these sprawling, time-worn sites were invisible in plain sight. But aerial observation technology now allows archaeologists to uncover patterns of land use and ancient architecture. Lidar systems use a grid of infrared beams, hundreds of thousands per second, shot downward from an aircraft. As each beam strikes something on the earth’s surface, it bounces back to provide a measure of distance. The resulting grid produces an enormous cloud of data points. Computer software crunches the data to create high-resolution images of the earth’s surface and structures on it, even scrubbing away trees in digital deforestation, which has been used to reveal cities lost in the Amazon.

By combing through several sets of lidar data, Šprajc and colleagues identified 415 distinct ceremonial complexes dating from 1100 B.C.E. to 250 C.E. Among them are the Olmec center of San Lorenzo, Mexico, and the recently discovered Aguada Fénix, on a Mexican ranch near the Guatemala border, which may be the biggest and oldest known Maya monumental complex. The group analyzed the sites’ astronomical orientations on notable days of the 260-day calendar, including solstices and lunar cycles.

Among the most common orientations found in the complexes dating to between 1100 to 750 B.C.E. were those aligned with sunrises on February 11 and October 29, two dates separated by the calendar’s full 260 days. Other intriguing connections appear to exist. Some sites, for example, seem to chart the seasonal extremes of Venus’ appearances as the “evening star,” which roughly bookend the region’s rainy season.

The complexes weren’t cities with large residential areas but functional observatory sites that also likely served as gathering places for important communal events or observations, the authors note. They were often arranged in the shape of a rectangle or square, and featured plazas surrounded by mounds, pyramids and platforms—themselves sometimes built according to key numbers at the core of the calendar system.

“Since the early orientations that we have analyzed [that] reflect the use of this cycle are embedded in the architectural complexes located along the southern Gulf Coast, this was most likely the area where the 260-day count originated,” says Šprajc.

However, not everyone is convinced by the findings. Gerardo Aldana, who studies Mesoamerican history, art and architecture at the University of California, Santa Barbara, notes that lidar is most effective when it’s followed up by investigations on the ground. “There simply isn’t enough of the latter to suggest that the authors are finding anything other than patterns in randomness within a sufficiently large data set,” Aldana says. And while the majority of the study’s approximately 400 sites face east and west, Aldana believes their orientations aren’t known precisely enough to support the study’s hypothesis. “In my view, the authors haven’t convincingly demonstrated that they have identified astronomically or calendrically oriented architecture, let alone specifically tied it to the 260-day count.”

The calendar, called Tzolk’in in Yucatec Mayan, has no months but features 20 different glyphs or signs, including crocodile, deer, water, grass and eagle, which are combined with the numbers 1 to 13, resulting in 260 unique days. The lidar discovery suggests that the calendar was in use about 800 years earlier than the oldest previous evidence for this cycle, the third-century B.C.E. “7 Deer” glyph found at San Bartolo in northeastern Guatemala.

“They’ve found all these really early sites sitting there in the cattle fields of Tabasco, that date from centuries before, anywhere from 1000 to 800 B.C.E., and have these orientations that signal an interest in the 260-day calendar,” says David Stuart, an archaeologist and Mesoamerica expert at the University of Texas at Austin who wasn’t involved in the research. “So I think it’s a really interesting, important paper that gives us this indirect evidence of the 260-day calendar in use.”

Stuart notes that the San Bartolo glyph, which he described last year in Science Advances, was found at a similar, though younger, site of astronomically oriented monumental architecture.

The 260-day cycle was one of the foundations of religion and cosmology shared by Maya, Aztec and other Mesoamerican cultures, Stuart explains. “It actually persists uninterrupted to this day in some pockets of Mesoamerica, used as a divination calendar kind of in the way astrologers would use signs, which is just incredible when you think about it.”

Stuart believes that the newly uncovered sites were functional observatories, but perhaps primarily places designed for ceremonial gatherings to mark such astronomical events as the rising of the sun on a notable day.

“This stuff is about building community, it’s at the core of Maya identity,” he says, “and we’re seeing it almost as early as we can trace the Maya themselves.”

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Brian Handwerk is a science correspondent based in Amherst, New Hampshire.