New research suggests that equinox alignments shaped the urban design of Cuicuilco, one of the earliest settlements in what is now southern Mexico City. Researchers from the National Institute of Anthropology and History (INAH) say the site’s layout was oriented toward the rising sun over Cerro Papayo, a hill to the east, during equinoctial mornings.
Archaeoastronomer Aarón Uriel González Benítez of the Astronomical Society of Iztapalapa led the study through the Department of Cultural Dissemination of the National School of Anthropology and History (ENAH).
He found that the sun’s apparent path over the horizon aligns with two periods of the pre-Hispanic Cempohuallapohualli calendar.
These periods, known as tozoztontli and teotleco, correspond to the spring equinox around March 24 and the autumn equinox around September 20.
Cuicuilco’s pre-Hispanic calendar and its equinox types
González Benítez noted that the globally recognized spring equinox falls on March 20 or 21, but ancient societies observed different types. The “middle” equinox marks the midpoint between solstices. The “cultural” equinox, by contrast, is defined by a group’s own calendar traditions based on the sun’s apparent movement across the horizon.
The main buildings of the ENAH (National School of Anthropology and History) are oriented towards the sunrise over Papayo Hill on the equinoxes. Courtesy: Aarón González. pic.twitter.com/yIwOQdhhm8
— Tom Marvolo Riddle (@tom_riddle2025) April 7, 2026
Buildings VI and VIII at Cuicuilco B face directly toward Cerro Papayo. Their square geometry aligns with the rising sun on the proposed cultural equinox dates of March 23 and 24. Academic sources have long identified that hill as a key landscape feature tied to solar observation at Cuicuilco A.
González Benítez explained that in pre-Hispanic culture, urban centers served as the axis of the world for their communities. Architecture, natural landscape, and solar movement worked together to mark the most significant moments of the cultural calendar.
Ancient equinox alignments echoed across modern Mexico
The pattern extends beyond the ancient ruins. Buildings on the ENAH campus, including its main building, annex, Guillermo Bonfil Batalla Library, and Research Tower, face between 89 and 90 degrees azimuthal east. The street grid of the neighboring Isidro Fabela district follows the same direction.
Researchers confirmed this overlap firsthand on March 23, 2026. González Benítez described the similarity between ancient and modern orientations as an extraordinary coincidence, one that lends the school an astronomical dimension it had not previously claimed.
José Manuel Castillo Hernández, head of ENAH’s Department of Cultural Dissemination, said the findings carry real educational value. He argued the discovery keeps local astronomical traditions alive and deepens the bond between the school and the archaeological zone beside it.
These ancient equinox alignments across Mexico, he added, also offer a meaningful way to bring pre-Hispanic knowledge directly into the classroom.
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