Archaeoastronomy. The Observation of the Skies in Prehispanic Mesoamerican Cultures

In the past, the firmament woke up intense feelings in humanity that went from religious reverence to the beginning of critical thinking, which finally led to scientific inquiry. This intellectual journey arose in Mesoamerica from a very early time with the Olmec civilization and it continued to develop for approximately three millennia. A fundamental aspect that derived from the systematic observation of celestial phenomena was undoubtedly the creation of a calendar system that made it possible to organize every social activity. 

The Mesoamerican calendar consisted of two day counts that ran simultaneously. One of them was the solar count—organized as 18 periods of 20 days each with five extra days. The other one consisted of 260 days and it was structured as 20 periods of 13 days each. Its origin was intertwined with religious and ritual aspects. After the first 260 days, both counts desynchronized forcing to wait for 52 years of 365 days to synchronize again, which was a very important calendar cycle. For these days to line up, the ritual count had to execute 73 periods of 260 days, thus establishing the calendar equation 52 (365) = 73 (260). 

The numbers that defined the calendar system’s qualities played a very important role in Mesoamerican culture. Architectural elements like the number of stair steps and bodies in pyramids, the amount of crenels in temples, and the amount of offerings were decided considering the numbers 13, 18, 20, 52, 73, and their multiples. An exceptional praxis in Mesoamerican architecture that took this into account was the selection of the orientation of the most important buildings in determined places. It is even possible to affirm that most of these Mesoamerican buildings align to sunrise and sunset on dates that differ from the solstice days by multiples of the calendar numbers we have seen. Two very striking examples are the pyramid of the Sun in Teotihuacan and Templo Mayor in Tenochtitlan. The dates of solar alignment, both happening around sunset, divide the year into two parts. The great 64-meter-tall pyramid in Teotihuacan points to the solar disc 52 days before and after the summer solstice. Another 260 days are required to complete the year. In the other example, the symmetry axis of the main Aztec temple points to the sunset on dates that happen 73 days before and 73 after the summer solstice. The complement to complete a full year is, in this case, three times 73 days. 

The relevance of these events takes us back to the social function of astronomy in societies of the past. Considering that our star reached the maximum status for religious veneration, associating these dates to an important temple represented a peculiar way of cult. The sky played the role of a great stage where the deity manifested itself at certain fundamental moments. According to the ideology of diverse Mesoamerican regions, the calendar was invented by a couple of long-lived deities that gifted it to humanity for its benefit. The ruling class that possessed calendar knowledge took the situation as an opportunity to link a significant architectural structure to the veneration of the gods through its alignment. This way, the gods would reward the rulers with favors that would make them remain in their powerful position. In that sense, astronomical knowledge contributed to a symbolic and political discourse of great meaning in society. This orientation praxis had been around for many centuries before both cities reached their respective peaks. This way of assuming the sky’s transcendence was already a vigorous part of Olmec and Mayan cultures from the Mesoamerican Southeast. 

Maya peoples of the Classical period reached the highest levels of observational precision. The Codex preserved in Dresden, Germany, contains records that report the regularity of solar and lunar eclipses, as well as the phases of Venus as the morning star and the evening star, determining the synodic period of the planet. These records are accompanied by hieroglyphic expressions whose concrete meaning is not always clear. While the inspiration for these observational calculus deeds could have been religious, this does not make them any less scientifically valuable. 

The record of astronomical phenomena was often made through artistic representation. A striking example can be found in a mural painting in Mayapan, Yucatán—big suns with descending and richly attired characters inside of them were captured. The wall faces the sunrise on the dates determined by the calendar count of seventy-three days. Before the astronomic question of what the characters could represent, it’s clear that the answer could be a sunspot or an inner planet. Sunspots do not always reach sizes big enough to be seen at simple view. Mercury is too small to be perceived without a telescope. Venus does have a bigger angular size than the smallest sunspot that can be seen with bare eyes, which suggests that it could be a Venus transit. Archeological determination of the time of creation leads us to the 7th and 8th centuries. Considering that Venus transits are periodical, two of them happened over that time during sunset. Since the horizon in the Yucatan region is mainly flat, it’s common to observe spectacular sunsets dampened by the low atmosphere with a red or orange Sun. The proposal is that the painting could record these transits of Venus. The transit of Venus—the Mayan god Kukulkan—in June 5, 2012, was clearly observed on the solar disc during sunset with bare eyes and no filters from the highest point of the Temple of Kukulkan in Mayapan. 

On certain occasions, celestial phenomena have determined historical moments. This could have happened in 1325 when Tenochtitlan was founded. The Aztecs had spent more than 200 years wandering, looking for the land promised by their solar god Huitzilopochtli. When they arrived at the Texcoco lake on April 21, they bore witness to a total solar eclipse at around noon; it became dark for over four minutes. According to the Nahuatl chronicles from the 16th century, an event as impactful for the senses could have been interpreted as a signal sent by the god to settle there, where the capital city of the Mexican Empire would flourish. By the moment Huitzilopochtli advised the Mexica they have reached the so longed place, an omen manifested in the waters of the lake—white colored fauna, caves from where colorful water sprung. Such an omen is compatible with the circumstances of the total eclipse—the aurora-looking shining across the horizon and reflected on the lake, the corona’s reflection and the chromosphere’s reddish gleams could explain such an omen. The only representation of the foundation scene that was carved in stone is on a monolith that was discovered in the center of Mexico City and called the Teocalli of the Sacred War. Shaped as a pyramid with 13 steps, it shows a vertical solar disc in its cuspid and the Mexica year 2-House which corresponds to 1325. The formal founding ceremony must have happened after this. Considering the calendar structure, several elements that suggest the importance of the 13-day-long period can be picked out from the described monolith. So, starting from the day of the eclipse—April 21—, after two 13-day-long periods, May 17 arrived—the precise day of the first zero shadow day in Tenochtitlan, the day when sun rays at noon fall vertically and objects cast no observable shadow as a result, meaning there would be a favorable day to pay tribute to the Sun god founding the city. This year marks the 700 anniversary since the founding of Tenochtitlan, today Mexico City. 

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