A recent study in Arqueología Mexicana is rethinking how we understand one of the most famous light-and-shadow displays at Chichén Itzá. For years, people have shown up around the spring and autumn equinoxes to watch what’s often called the “descent of Kukulcan.”
As the sun sets, shifting and shadow along the pyramid’s northwest staircase create a shape that looks like a serpent slithering downward. It’s usually described as something that happens precisely on the equinox.
The new research suggests it’s not that simple.
The team—Orlando Casares Contreras, Arturo Montero García, Jesús Galindo Trejo, and David Wood Cano—found that the effect develops over a much longer stretch of time. Instead of a single-day alignment, it appears to be part of a broader, carefully structured sequence tied to the sun’s yearly path.
The pyramid itself, known as El Castillo, stands about 30 meters tall with nine stepped levels and stairways on all four sides. At sunset, shadows falling across the northwest balustrade form a series of triangular shapes. As these triangles connect, they create the familiar serpent-like illusion.
According to the study, the process begins as early as February 12, when a faint triangle appears near the top. At first, it’s barely noticeable and doesn’t last long. Over the next few weeks, more triangles show up one after another, though they still fade before sunset.
By early March, the pattern becomes easier to see. Around March 4, five triangles are visible. A few days later, there are six. By March 15, seven triangles appear and remain visible all the way through sunset—a pattern that holds for about ten days, including the spring equinox.
So the equinox isn’t a standalone moment. It falls within a wider window. The researchers think the appearance of seven triangles may have been a useful marker for ancient observers. Shortly after, an eighth triangle shows up, signaling that the period has shifted.
The sequence keeps building. By April 9, there are nine triangles, matching the pyramid’s nine tiers. As weeks pass, the shapes widen until, by May 24, the entire balustrade is lit up. That date lines up with the sun’s first zenith passage in the region, when it sits directly overhead at midday.
The full illumination continues through the summer solstice in June and briefly returns around July 19, during the second zenith passage. After that, the pattern gradually reverses.
In early September, the nine triangles appear again before shrinking. Around the autumn equinox, the seven-triangle pattern comes back, mirroring what happens in spring. By October, the number drops to five, and by the end of the month, the effect nearly disappears.
For several weeks after that—around the winter solstice—the staircase stays mostly in shadow. Then, in mid-February, the cycle starts over.
The researchers argue that this slow, repeating pattern would have been more practical than focusing on a single date. Watching how the light changed over time could have helped track seasons and plan agricultural work. It also fits with other Mesoamerican structures that align with key solar events.
Rather than relying on one exact day, the Maya may have paid attention to a sequence of visual cues. In that light, El Castillo isn’t just a monument—it’s a working system, combining architecture and astronomy to measure time in a way that’s still visible today.
Sources : INAH