The role of the astronomers in the Mesoamerican societies was quite considerable, and we do have clear evidence that astronomers were involved in the planning of monumental buildings as well. Orientations in Mesoamerica concentrate at the solstices and at the days, very close to the equinoxes, marking quarter days of the year, midpoints in time between the solstices; moreover, the intervals separating sunrise and sunset dates recorded by orientations, tend to be multiples of 13 or 20 days. The urban layouts are skewed clockwise from cardinal directions by an amount usually close to 17°, a tradition originated at Teotihuacan and probably indicating an interest in the Sun rising in autumn and/or setting in spring. As an example of what the Mesoamerican architects were able to realize, we shall visit today a famous post-classic Maya town of the Yucatan. The Yucatan peninsula is dotted with peculiar natural features called cenotes. A cenote is a sinkhole which results from the collapse of the bedrock surface and is usually filled by groundwater. Cenotes were considered sacred by the Mayas, who used them as gateways of spiritual communication with the other world and, as a consequence, cast sacrificial offerings, including humans, into them. It is probably on account of the presence of two imposing cenotes that Chichen Itza was founded. Around the tenth century AD, it became one of the most important city of Maya world. The architecture of Chichen Itza is famous for many items, including a huge ballcourt – the biggest of the maya world – and the so-called Caracol, an enormous structure with circular layout, built over a huge platform. From the outside it resembles an astronomical observatory covered by a domed vault, but the dome is actually almost filled in, apart from a spiral passage that gives the building its name ‒ Caracol means snail in Spanish. There are many particular features of the edifice, which are likely to correspond to intended astronomical alignments, but we cannot be sure that astronomical observations were really carried out from here. In any case, the interest in astronomy by the builders of Chichen Itza is apparent, when we move to the most impressive of its monuments, the world-famous pyramid usually called Castillo. It is a square building 24 metres tall and each side is fitted with a staircase which reaches the temple on the summit. The monument is dedicated to Kukulcan, the Plumed Serpent, a very ancient divinity in Mesoamerican religion. Two monolithic sculptures of the serpent's head are placed on each side of one of the staircases; clearly this is the most important one, as it leads to the main entrance on the summit. The building incorporates an older pyramid, with its summit temple still intact, containing a Chac-Mool ‒ a statue of a recumbent man which was used for human sacrifice ‒ and a jaguar-shaped throne. Temples were usually overbuilt in calendrical occasions, for instance the occurance of “calendar round” dates, and indeed, the relationship of the monument with calendrics is very strong. First of all observe that the square base is skewed 22° clockwise with respect to the cardinal directions. By analysing the motion of the Sun at Chichen Itza, it can be easily verified that in this way the azimuth of the façade which points to the north of west, fits remarkably well with the setting of the Sun on the days of the zenith passages, at this latitude the 20th of May and the 24th of July. The interest in the passage of the Sun at the zenith is, on the other hand, very well documented in Mesoamerica; it was studied with appropriate devices that enabled astronomers to recognise the transit of the Sun directly overhead. But the most striking astronomical aspect still await us. The Castillo was in fact designed as a three-dimensional astronomical device, and this feature makes this wonderful monument quite unique. To understand how the device works, let us observe that, in general, the shadow produced by the north-west corner of the pyramid in the afternoon will be projected onto the north-east face in different ways on different days and at different hours. Generally speaking, the corresponding staircase ‒ the one bearing the serpent heads ‒ could either be in shadow or illuminated. Only as a result of very special positions of the Sun ‒ and therefore on special days and hours ‒ it will be half-illuminated, that is to say, the undulating profile of the corner will project a undulating figure onto it. When this occurs, a light and shadow effect connects with the serpent head at the base, creating the image of the descent of a giant snake. The monument has been designed in such a way that this phenomenon takes place ‒ reaching its maximum about one hour before sunset ‒ in the days close to the equinoxes; it was however certainly meant to be seen at or near the Spring equinox in March, as September is in full rainy season while March marks the arrival of it. This calendrical function is therefore, clearly symbolic: the serpent descends the Castillo stairway as a harbinger of the rainy season. Where is he going? The direction indicated by the stairway where the hierophany occurs, crosses another monument of the esplanade, the so-called Venus Platform. Nearby starts a sacbé, a ceremonial causeway which today can still be run up to the largest of the Chichen Itza cenotes, the so-called Well of the Sacrifice. The sacred water here is thus the final destination of the god. This appointment ‒ fixed by the Maya one thousand years ago ‒ was forgotten and only rediscovered in the first half of the last century; nowadays thousands of people gather each year at Chichen Itza to see it.
