Mesoamerican Calendars

Mesoamerican calendars are the calendrical systems devised and used by the pre-Columbian cultures of Mesoamerica. In addition to the basic function of a calendar—defining and organizing periods of time in a way that allows events to be fixed, ordered and noted relative to each other and some absolute progression—Mesoamerican calendars were also used in religious observances and social rituals, such as for divination.

Among the various calendar systems in use, two were particularly central and widespread across Mesoamerica. Common to all recorded Mesoamerican cultures, and the most important, was the 260-day calendar, a ritual calendar with no confirmed correlation to astronomical or agricultural cycles. Apparently the earliest Mesoamerican calendar to be developed, it was known by a variety of local terms, and its named components and the glyphs used to depict them were similarly culture-specific. However, it is clear that this calendar functioned in essentially the same way across cultures, and down through the chronological periods it was maintained. The second of the major calendars was one representing a 365-day period approximating the tropical year, known sometimes as the "vague year". Because it was an approximation, over time the seasons and the true tropical year gradually "wandered" with respect to this calendar, owing to the accumulation of the differences in length. There is little hard evidence to suggest that the ancient Mesoamericans used any intercalary days to bring their calendar back into alignment. However there is evidence to show Mesoamericans were aware of this gradual shifting, which they accounted for in other ways without amending the calendar itself.

These two 260- and 365-day calendars could also be synchronised to generate the Calendar Round, a period of 18980 days or approximately 52 years. The completion and observance of this Calendar Round sequence was of ritual significance to a number of Mesoamerican cultures.

A third major calendar form known as the Long Count is found in the inscriptions of several Mesoamerican cultures, most famously those of the Maya civilization who developed it to its fullest extent during the Classic period (ca. 200–900 CE). The Long Count provided the ability to uniquely identify days over a much longer period of time, by combining a sequence of day-counts or cycles of increasing length, calculated or set from a particular date in the mythical past. Most commonly, five such higher-order cycles in a modified vigesimal (base-20) count were used, generating a linear progression of days to span a period of roughly 5125 solar years.

The use of Mesoamerican calendrics is one of the cultural traits that Paul Kirchoff used in his original formulation to define Mesoamerica as a culture area. Therefore the use of Mesoamerican calendars is specific to Mesoamerica and is not found outside its boundaries.

The existence of Mesoamerican calendars is attested as early as ca. 500 BCE, with the essentials already appearing by then as fully defined and functional. Mesoamerican calendar usage—of one form or another, some (such as the Long Count) fell into disuse earlier—continued throughout the pre-Columbian era until the arrival of the Spanish conquistadores in the early 16th century. Even after the subsequent colonialisation of Mesoamerican territory by Europeans and the consequent adoption of the Julian Calendar, some indigenous communities continue to use aspects of Mesoamerican calendars in parallel with the Western system, such as among K'iche Maya communities of the Guatemalan highlands and the Mixe of Oaxaca.