Twice a year, in the spring and in the fall, the feathered serpent god Kukulcan crawls down the side of the great pyramid named after him (or as the Spanish named it, “El Castillo,” the castle). The angle of the sun glinting off the northwestern corner of the pyramid projects a shadow on its north staircase that resembles a wriggling serpent, made even more explicit by the carved serpent head at the base of the staircase.

The light-and-shadow phenomenon is occurring as we speak, and will reach its peak tomorrow (Sept. 22) on the fall equinox. Because this is the rainy season, visitors find it more hit-and-miss to catch the effect, but still many hundreds show up hoping to see it.

You can learn more about how the Maya configured the pyramid to take advantage of the sun HERE. And here is a 10-minute video shot six months ago at the spring equinox: