Zenithal passages of the Sun at Monte Alban, Teotihuacan and Xochicalco

This article is a joint effort of recognition of three Mexican World Heritage sites that share in their past the important legacy of knowledge of skywatchers in ancient Mexico.

Zenithal passages of the Sun at Monte Alban, Teotihuacan and Xochicalco
Zenithal steps of the Sun. Image by Adina Voicu from Pixabay

Talking about astronomy in pre-Hispanic times may evoke the image of ancestors observing the stars at different times of their lives, that is, both day and night; during sunrises or sunsets; but we must also consider that this was done during such a trivial time of the day as in the case of this article, noon when the zenithal passage of the sun is observed.

In the Mesoamerican world the great giver of life, the Sun, manifested itself in multiple ways according to the perception or the interests of those who observed it. Some followed it by observing daily the moment of elevation above the horizon (solar ortho); others, on the contrary, at its setting below the horizon (solar twilight), from where the sun would resume its journey below the world of humans, hidden from their gaze during the night, which sheltered its journey below the celestial regions.

Some followed the sun during each day to determine the moment of its culmination from where it began its gradual descent towards the place of humans; among those days there were some exceptional ones, those in whose solar culmination, shadows were not projected, that is, in the zenithal passage of the sun or simply the zenith of the sun. In Mesoamerican thought, several ways of following these events were devised, although we do not know if any explanation was given as to how they happened.

Thus, there is evidence of zenithal observations of the sun in different places of that Mesoamerican world of fully developed civilizations. In the Mayan area, this refers to a pre-Hispanic group of towers located in Nocuchich, Chenchan, Tabasqueño, and Hochob, which did not cast a shadow on the days of solar zenith; another important tower is that of Palenque, and course, the vertically placed Mayan stelae, or the crests of some buildings, must also have served as zenithal passage markers.

In other sites, besides the last elements of the material culture, the sunken patios could have been used to multiply the illumination to the interior and to make observations, as the existing ones in many sites like in Monte Alban, or perhaps being great spaces like in the Citadels. Another possibility was in altars of different dimensions located in plazas or interior patios.

Of course among these elements of the material culture are the subway enclosures built to observe the opposite effect of a zenithal passage, that is to say, the incidence of a beam of light to the dark interior of an observatory to illuminate its interior with certain luminous intensity, examples of this are the one of Building P of Monte Alban, the Observatory of Xochicalco and the ceremonial cave with a smooth stela in Teotihuacan.

The zenithal passage of the sun

Given someplace in the Mexican Republic, it is possible to imagine a vertical line, projected towards the celestial vault, which points to a point on it, called the zenith of the place. When the sun, according to its apparent movement during the year, passes through that point, that day is called the day of the zenithal passage of the sun, a day that will also be for all populations that have similar latitudes. On the other hand, we must also keep in mind that the sun passes twice in the year through the zenith, so we will have two dates in the year for any geographical location with latitude lower than that of the Tropic of Cancer (of the order of 23.44º at present).

The Yearbook of the National Astronomical Observatory for the year 2021 can be consulted, where the zenithal passes of the sun for some populations of the Mexican Republic are published on pages 204 to 207. Next, three cases of pre-Hispanic spaces destined to the observation of the zenithal passage of the sun in sites that today are part of the Mexican World Heritage are described, such as the Archaeological Site of Monte Albán inscribed in 1987, the Zone of Archaeological Monuments of Xochicalco inscribed in 1999, and the Pre-Hispanic City of Teotihuacán inscribed in 1987.

Monte Alban Pre-Hispanic Observatory

Returning to the previous idea, the zenith sun is the position of the sun over the vertical of a place (zenith) at noon. The zenith is the point of the celestial sphere that is vertically over the observer's head, that is, it forms an imaginary line with an angle of 90 degrees for the horizon.

During the zenith passage, the sun occupies the highest place in the sky and radiates its energy more intensely on the earth's crust. The zenithal passage only occurs in the regions between the Tropic of Cancer and the Tropic of Capricorn, two days a year, during which no lateral shadow is cast.

The dates differ according to the latitude of each place, so the sun illuminates "plumb" different areas of the planet on different dates. In the specific case of Oaxaca, this event occurs both on May 8 and August 5.

In pre-Hispanic times, those who knew about the sky, aware of this phenomenon, gave it great importance. Archaeological evidence of its importance was found in different archaeological sites such as Xochicalco and Teotihuacan, where its inhabitants built special observatories to appreciate it.

In the case of Monte Alban, the structure called Building P has a dark chamber inside, which, thanks to a chimney, is illuminated during the zenithal sun. A possible interpretation is that this phenomenon is related to the beginning of the rainy season and, with it, the planting of the milpa, one of the bases of Mesoamerican civilizations.

Prehispanic observatory of Xochicalco

During the study of Mesoamerican civilizations, one is surprised by the great number of findings that let us learn about the great conceptual structure of astronomy, which we can notice in the elements of the material culture of Xochicalco. An example of this is the Pyramid of the Feathered Serpents with its calendrical records, or the patio of the Stela of the Two Glyphs from where the sunrise and sunset were observed at the equinoxes, also the day of the beginning of the annual count in the calendar, as well as the zenithal events of the Sun, the Moon and planets through the pre-Hispanic zenithal observatory.

In the case of the Sun at the zenith, it was of great importance that gathered a large number of scholars in the subway enclosure, around May and July, to verify the count of the days. Thus, the observatory of Xochicalco is a transcendent signifier that shows us the power of Mesoamerican thought, in formulating an idea of how to solve the observation of solar events that were repeated ad infinitum.

Astronomy is the study of the different stars that we can observe thanks to the biggest and best instrument of specialized observation that we have with us as human beings, which is nothing more and nothing less than our sight. With which, our ancestors contemplated, but also studied and investigated in detail in an exact way all the movements and trajectories of the sun, the moon, some planets, the stars, comets, and even the Mesoamerican people understood with great precision that the apparent movements of the stars are caused by the movements that the earth has naturally, as is the case of rotation and translation.

The indigenous city of Xochicalco, bastion of pre-Hispanic astronomy, was built more than 1300 years ago, most probably by Teotihuacan emigrants who, without a doubt, brought with them knowledge, cultural traditions, and different knowledge compiled in the great Teotihuacan, and perhaps from there they maintained contacts with the Mayan area, the area of Oaxaca and the Gulf Coast, to convene in Xochicalco during a total eclipse of the sun to the time counters, the characters in charge of keeping track of the calendar and seek a unification of the same.

The house of the flourishing knowledge preserves within its architectural spaces, the clear and indisputable use of astronomical knowledge for the layout of the city or the orientation of the axes of the main buildings, but especially highlights three significant spaces that reflect one of the exceptional universal values of this place that today is part of the World Heritage List, the Temple of the Feathered Serpents, the Plaza of the Stela of the Two Glyphs and the Pre-Hispanic Observatory.

The Pre-Hispanic Observatory of Xochicalco is a cave that was formed naturally by a chemical and physical process that has to do with the composition of the rock that forms the Cerro Xochicalco, a cave that was used to extract raw material for the construction of the city and finally was conditioned to perform accurate specialized astronomical observations.

The two zenithal passes of the sun in the pre-Hispanic Observatory of Xochicalco are appreciated between May 14 and 15 and between July 28 and 29 at noon, with a total estimated duration of 49 minutes, optical phenomenon that, when being observed inside a cave that in turn works as a dark chamber, highlights the magnitude and beauty of this optical effect, being able to be illuminated at the moment of greater concentration of light, from the shot to the current entrance to the observatory.

Prehispanic Observatory of Teotihuacan

To the above-mentioned observatories, we can add the vertical shaft located in front of the stairway of the Feathered Serpent Pyramid in Teotihuacan, whose discovery occurred at the end of 2003 in the scope of the projects of Julie Gazzola and Sergio Gomez de la Ciudadela in Teotihuacan.

The participation of the astronomer Daniel Flores Gutiérrez, invited by the archaeologists to their projects, consisted of descending the vertical shaft and realizing that in the Teotihuacan past this vertical shaft could have been used as a zenithal observatory, in addition to other uses that must have been applied daily during certain intervals of time, of the order of lustrums or decades. On one occasion when the protective cover of the vertical shaft was removed, it was possible to obtain the image of the incidence of the sunlight beam towards the base of this shaft, which is tangent to the tunnel that reaches the center of the Pyramid of the Feathered Serpent, located about 12 to 15 meters below the level of the plaza.

Similarly in Teotihuacan, we have a volcanic formation cave, which was studied by Enrique Soruco, discovered in the season of explorations from 1980 to 1982, with the great quality that in addition to registering the zenithal steps, it indicates the incidence of sunlight from February 9 to November 2 approximately, indicating its use in the control of some calendar account. The registration of the first zenithal passage of the sun in the astronomical observation spaces in Teotihuacan occurs on May 19.

As we could appreciate, this article is a joint effort of recognition of three Mexican World Heritage sites that share in their past the important legacy of knowledge of the observers of the sky in ancient Mexico, expressed through the adaptation of natural caves and in some cases the construction of specialized spaces for the observation of the day and the hour in which the sun does not cast shadows. It is very important to recognize that the National Institute of Anthropology and History, responsible for the care of these spaces, continuously carries out diverse conservation and maintenance tasks so that these places are preserved for the benefit of humanity.

Let us know, let us remember, and let us once again give value to astronomical observation as did the builders of the great indigenous cities that are part of our identity.


Jesús Medina Villalobos, Monte Alban Archaeological Zone

Architect. José Cuauhtli Alejandro Medina Romero, Director of the Archaeological Zone of Xochicalco

Dr. Daniel Flores Gutierrez, Institute of Astronomy, UNAM

Dr. David Andrade Olvera, Director of the Archaeological Zone of Monte Albán, Source: INAH