Teotihuacán was the largest urban center in pre-Columbian America and, in the
hey-day of its existence, one of the three largest cities in the world, rivaling Rome
in Europe and Beijing in Asia. It is located in the Valley of Mexico, a highland basin
on the Mexican plateau which was centered on a large, shallow lake known as
the "Lake of the Moon", about 50 km (30 miles) northeast of modern Mexico City.

The city was founded about 150 B.C. by refugees from the ancient "Olmec"
ceremonial center of Cuicuilco, which lay near the southwestern edge of the lake.
An eruption of the volcano Xitle had sent the residents of Cuicuilco fleeing around
the lake to the northeast, where they laid out their new metropolis according to
a meticulously gridded "grand design". When archaeologists began seriously
excavating the site in the 1960's, they found that the entire city and its environs
had been oriented along axes that varied 15.5º from the cardinal points. Thus, the
the principal thoroughfare of the city -- called by the Aztecs "The Street of the
Dead" -- began at the Pyramid of the Moon and ran south-southwestward along an
azimuth (i.e., compass bearing) of 195.5º, or 15.5º west of south. (See photograph
above.) All major structures, such as the gigantic Pyramid of the Sun (seen in the
middle background of the photograph above and from its western front in the
photograph below), and all cross-streets were aligned at right angles to this in turn,
meaning that they were oriented to an azimuth of 285.5º, or 15.5º north of west.

Visitors to the museum at Teotihuacán are informed that the city owes its
layout to the azimuth at which the suns sets on the days that it passes directly
overhead at the site, namely July 26 and May 18. Yet, when the sunset azimuths
are calculated for those dates, it will quickly be seen that they occur at 290.7º
and not 285.5º as the city master-plan itself reveals!

In 1975, when I put my students to work calculating what "sunset" the city's
orientation actually commemorated, they were quick to come up with an answer
that proved immensely exciting -- August 13th, "the day the world began"
according to the ancient Mesoamericans! Here, on the Plateau of Mexico, city-
planners had built a ceremonial center with a configuration commemorating a date whose
calendrical importance had first been recognized 1000 km (600 miles) away to
the south more than a millenium earlier. And, in addition, they had located the
city with such precision that it was also aligned to the highest mountain in
all of Mexico at sunrise on the winter solstice --Citlaltépetl, or Orizaba (5700 m,
or 18,700 ft. in elevation). To be sure, a low ridge obscures the mountain from
the direct view of anyone standing atop the Pyramid of the Sun, but the alignment
is so exact that in a paper published in 1978 I hypothesized that a "relay station"
of sorts must have been constructed on the intervening ridge to alert the priests
of the solsticial sunrise. In January, 1993, with the help of a GPS (i.e., global position-
ing system) receiver I managed to confirm that such a "station" had in fact existed.

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