A new study has shed fresh light on one of the ancient world's most celebrated architectural wonders. Four archaeoastronomers from leading Mexican research institutions have demonstrated that the famous "descent of Kukulcán" light phenomenon at El Castillo, the great pyramid at Chichén Itzá, is not confined to the equinoxes, as was long believed, but follows a precise, year-round pattern of solar illumination that encodes the entire Maya calendar within its very stones, . The findings, presented in Arqueologia Mexicana, suggest the pyramid was designed as a functioning astronomical instrument, capable of marking every major celestial event of the solar year.
The Mechanics of the Serpent of Light
The pyramid known as El Castillo, or the Temple of Kukulcán, stands 30 meters (98 feet) tall and features nine terraced bodies topped by a temple sanctuary. Each of its four sides measures 55.5 meters (182 feet) and is flanked by stairways with balustrades. As the sun descends toward the western horizon, it casts shadows from the pyramid's corners across the northwest ramp, forming a sequence of triangular patches of light that together trace the undulating body of a serpent — the hierophany known as the "descent of Kukulcán."
According to the study's authors; Orlando Casares Contreras of the Centro INAH Yucatán, Arturo Montero García of the Universidad del Tepeyac, Jesús Galindo Trejo of the UNAM's Instituto de Investigaciones Estéticas, and David Wood Cano of the Seminario de Arqueoastronomía ENAH-UNAM, the process begins as early as February 12, when the first faint trace of light appears on the upper portion of the ramp. At this stage, the triangles are incomplete and last only a few minutes. By March 4, five triangles are visible in the late afternoon, and by March 15, seven triangles of light are projected onto the balustrade, remaining visible until sunset.
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The serpent effect on the staircase of El Castillo at Chichén Itzá, demonstrated by artificial light (Bjørn Christian Tørrissen/CC BY-SA 3.0)
The researchers note that the actual astronomical equinox falls precisely in the middle of this seven-triangle window, five days after the pattern first forms and five days before the eighth triangle appears on March 26. This finding leads the team to propose that the seven-triangle display may have served as a practical method for Maya priest-astronomers to identify the equinox with remarkable accuracy - a conclusion that challenges some scholars who have doubted whether Mesoamerican observers ever formally identified the equinox.
Intervals generated by the orientation of El Castillo, showing the solar year divided by zenith passages, equinoxes, and solstices. (David Wood Cano / Arqueología Mexicana)
Marking the Zeniths, Solstices, and the Full Solar Year
The astronomical display does not end with the equinox. By April 9, all nine triangles are fully formed — one for each of the pyramid's nine terraced bodies. The triangles gradually widen over the following weeks until May 24, the date of the first solar zenith passage at Chichén Itzá, when the entire balustrade is bathed in light. This full illumination persists through the summer solstice on June 21 and concludes with the second zenith passage on July 19. Following the second zenith, the nine triangles begin to redefine themselves, reaching their complete form once more by September 2, before the pattern reverses as the autumnal equinox approaches. The last brief flashes of light are observed around October 29, after which the ramp remains in shadow for exactly 52 days before the winter solstice, and another 52 days after it, until the cycle restarts on February 12.
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El Caracol, the Maya astronomical observatory at Chichén Itzá (CC BY-SA 2.5)
The significance of the number 52 in this context is not coincidental. The 52-year Calendar Round — the period after which the Maya 365-day solar calendar (the Haab') and the 260-day ritual calendar (the Tzolk'in) realign — was one of the most sacred cycles in Mesoamerican timekeeping. The shadow-free interval of 52 days on either side of the winter solstice appears to encode this fundamental unit directly into the pyramid's design. The archaeoastronomy embedded in El Castillo thus operates simultaneously on multiple calendrical levels, linking the daily movement of the sun to the deepest cycles of Maya sacred time.
A Masterpiece of Calendrical Architecture
The mathematical elegance of El Castillo's design is laid bare in the study's analysis of key numerical intervals. The interval between the solar zenith passages and the temporal equinoxes is 63 days, while the interval between the temporal equinox and the solstice is 28 days. Together, these two intervals sum to 91 days - the number of steps on each of the pyramid's four stairways. Multiplied by four and adding the temple platform as the final step, the total reaches 365, the number of days in the solar year. The numbers 7 (the triangles that mark the equinox), 9 (the pyramid's terraced bodies), and 13 (a sacred Maya number) interlock through the relationship 63 = 7 × 9 and 91 = 7 × 13, revealing a system of extraordinary internal coherence.
The researchers conclude that El Castillo served as the axis mundi of the Maya world, connecting the celestial realm (symbolized by the number 13) with the underworld (represented by the number 9) through the surface of the earth where the serpent of light descends to fertilize the soil. The tracking of this illumination pattern throughout the year by Maya priest-observers would have provided not only a means of locating the equinox, but also a reliable guide to the agricultural and ceremonial calendar that structured Maya civilization for three millennia.
As the study's authors write, "the Maya made use of their astronomical knowledge, encoded in calendrical counts, to achieve admirable precision in the measurement of time."
For further reading on the site's many secrets, see also Hidden Passage Discovered Underneath Chichén Itzá and How Ancient People Marked the Equinox Around the World.
Top image: The pyramid of El Castillo (Temple of Kukulcán) at Chichén Itzá, Yucatán, Mexico. Source: Stacey MacNaught/CC BY 2.0
By Gary Manners
References
Casares Contreras, O., Montero García, I. A., Galindo Trejo, J. & Wood Cano, D. 2026. El Castillo de Chichén Itzá. Evocación de un mensaje majestuoso de trascendencia calendárica. Arqueología Mexicana, núm. 197, pp. 78–87. Available at: https://arqueologiamexicana.mx/mexico-antiguo/castillo-chichen-itza-calendario
INAH (Instituto Nacional de Antropología e Historia ). 2026. Aspectos calendáricos y astronómicos de El Castillo de Chichén Itzá, en Arqueología Mexicana. Boletín 180, 28 April 2026. Available at: https://inah.gob.mx/boletines/aspectos-calendaricos-y-astronomicos-de-el-castillo-de-chichen-itza-en-arqueologia-mexicana
