The Tropic of Cancer is the northern parallel at which the sun passes to the zenith on one day only, the day of the summer solstice. If we consider, instead of the movement of the rising point of the sun on the eastern horizon throughout the year, the movement of an observer who watches the maximal height of the sun at midday during the year shifting each year further north, we can think of the tropic as a zenith station, a point at which the movement of the shadow projected by the post of the observer finally reaches zero only once a year, at the summer solstice. The tropic is therefore a limit from which, descending again toward south, the shadow "turns back.'' I have produced such an utterly complicated reasoning because it should help to understand why the Teotihuacan astronomers moved toward north, and it is indeed certain that they did this.
The Tropic of Cancer passes through the north of Mexico, very close to the top of a hill known as El Chapin. Nestling in the foothills, on a parallel less than three arc minutes from the tropic and on the same parallel as a hilltop known as Picacho Peak, about 8 kilometers away, we find Alta Vista, one of the heartlands of the so-called Chalchihuites culture. First studied by Manuel Gamio in 1908, the site, which displays very clear Teotihuacan influence, was subsequently investigated in depth by J. Charles Kelley beginning in 1970. What Kelley wished to understand particularly was why the city had been built on an open plain that was impossible to defend (while various other settlements of the same period sprang up on hills or mountainsides) and that was over 2 kilometers from the nearest river.
The first construction to be built at Alta Vista was the Hall of Columns. This structure, with its square layout oriented to the cardinal points, was designed and executed with the utmost care, in such a way as to align a diagonal with the parallel that crosses Picacho Peak. The deliberate link with
the sky is confirmed by the fact that, near the north corner of the hall, Kelley found a multiple burial place, possibly of human sacrifices, with four ceramic pots positioned at the cardinal points. The pots allude to the divine cult that the Aztecs were to call Tezcatlipoca, a deity associated with the sky and the north. Another structure known as the Labyrinth is also aligned with the cardinal points. It consists of a series of pillared corridors, constructed in such a way that the equinoctial sun, coming up over Picacho Peak, passes over it without encountering any obstacles whatsoever. The spectacle to be enjoyed (it can still be appreciated today) from Alta Vista during the equinoxes is truly breathtaking; it is almost as if the sun wants to stay hidden behind the peak of Picacho and then pass over the city in triumph.
The Hall of Columns and the Labyrinth are among the few known examples in Mesoamerica of buildings aligned with the cardinal points, an indication perhaps of the fact that the city was somehow associated with them. It would seem, therefore, that, rather than being a direction that was somehow privileged or important at every single point, the east-west cardinal direction was identified with a specific parallel, the one at which the solar zenith has a "station", that is, the Tropic. Naturally, at Alta Vista the solstitial direction—recall that at the tropic it coincides with that of the zenith passage of the sun—was carefully measured as well, as is shown by the alignment, also very precise, between the top of Picacho peak and two pecked crosses, situated near the summit of Cerro Gordo.
All in all, Alta Vista was likely founded for strictly astronomical-symbolic (or religious, if you wish) reasons by people, probably guided by a group of Teotihuacan astronomers, who moved north in order to find the place where the sun "turned back," and to build a city there (Aveni et al. 1982). How did they manage to locate the Tropic with such amazing accuracy? There are only two possibilities, in my opinion. One is by measuring the direction of the rising of the sun at the zenith passages at regular distances, shifting successively north, and the other is by taking measures of the height of the sun on a fixed day, preferably at the solstice. Either way is a long and tricky endeavor. I thus believe that they may have developed a model that allowed them to cut down on the number of measurements required. It is certain, in any case, that as the number of days between the two zenith passages diminished, it was necessary to increase the number of measurements (or observers positioned in various places); otherwise it would have been impossible to locate the tropic so accurately.
The Teotihuacan astronomers who determined the position of the tropic of course knew that the number of days that elapse between the two zenith passages increases if one moves south. Maybe, therefore, they went to look for the equator as well. Certainly, someone was looking for it, and this someone was an excellent astronomer. The equator crosses not far from Quito, Ecuador. Today, tourists go to Mitad del Mundo, a village that has made a fortune out of a white line traced on the asphalt and a monument baptized the equator, to take photos of themselves with one foot in one hemisphere and one foot in the other. But whoever worked out where to draw the white line with modern instruments made a mistake: the equator does not pass through there at all, but several hundred meters to the north, on the parallel of the summit of Monte Catequilla. Here, are to be found the ruins of a pre-Columbian building, definitely pre-Inca but of uncertain date, which lies, believe me or not, exactly on the line of the equator.
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