On the day of the vernal equinox, crowds gather in Chicen Itza, in Yucatan. This is a sort of millennial rendezvous, which had long been forgotten, and was only rediscovered in the 1940s by photographer Laura Gilpin (Carlson 1999).
The monumental center of Chicen is dominated by the Castillo, a stepped square-based pyramid, about 25 meters wide and inclined at 45 degrees. A flight of stairs, each with 91 steps, climbs up each side, and there is a small structure on top. The building was dedicated to the plumed serpent, its head protruding from the base of the stairs (the pyramid was built in the Toltec-influenced period around the ninth century AD, encompassing a smaller preexisting Mayan building), and is oriented in such a way that one of its diagonals points northwest. This direction (roughly) identifies the rising point of the sun in the days of the zenith passages and consequently on these two days two faces of the pyramid remain in complete darkness until the sun passes the meridian.
The Castillo thus acts as a stone calendar, recalling the duration of the Haab year with its 364 (91 x 4) steps and platform on top, and indicating the zenith passages with its shadow. But the building has a further astronomical function: marking the equinoxes. The way it does this is quite unique. If one examines the outline of the edges of the pyramid, one notes that they are somewhat rounded off, sinuous. The reason for this is that its shadow is therefore rendered sinuous, serpent-like. This shadow is normally "lost'' in the side of the pyramid. Yet the dimensions of the stairs and of the
outline of the edges were precisely modeled in such a way that, roughly half an hour before sunset on the days near the equinoxes, the shadow is projected along the stairs. The result is that the shadow is magically seen to trace the silhouette of the body of a giant snake, which ingeniously connects with the stone head at the base of the pyramid.
The plumed serpent thus turns up punctually, twice a year, for his millennial rendezvous.
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