At the same time as Teotihuacan, but further south, in the Oaxaca valley, the culture known as Zapotec was evolving. Its main center was Monte Alban, which fortunately has remained almost intact. It is a stunning, enormous site, second only to Teotihuacan. It is in fact extremely ancient (10th to eighth century BC), but the structures visible today are those built by the Zapotecs. The complex is made up of great squares, pyramids, and a field for ball games, and there is an overall sense of harmony, of parallel lines and privileged directions, as at Teotihuacan. The orientation of the city too is Teotihuacan-style—on average 4 to 8 degrees east of true north, with only one conspicuous exception: a strange asymmetrical construction that seems to have been built carelessly, with ungainly, lopsided angles. It is called Building J and was erected in three successive stages, starting from the third century BC. The layout is equally curious: it resembles a shield, with one end facing southwest, and, as far back as 1935, the archaeologist Alfonso Caso suspected that it might have something to do with astronomy. Now we know that the monument is associated with astronomical observation, indeed the oldest of this type known in Mesoamerica.
The structure has two main alignments. The southwest tip faces the region where the stars of the Southern Cross were setting at around 250 BC, while the flight of steps to the north is directed toward the rising of Capella, one of the brightest stars in the sky, in the same period. At Monte Alban, Capella had the same function as the Pleiades at Teotihuacan (which lies about 500 kilometers further north), that is, it had heliacal rising on the same day as the first zenith passage of the Sun (Aveni and Linsley 1972). Building J is not unique; there is a close copy at the site of Caballito Blanco,
50 kilometers from Monte Alban, that is oriented toward the setting of Sirius.
Not far from Building J, and ideally linked to it by astronomical alignments that pass overhead, is another structure, Building P. Inside is a chamber connected with a vertical shaft with a hole in the top. Due to its reduced size, the hole offers a somewhat restricted view of the sky and was thus probably used as a device—a "zenith tube''—indicating the days around those of the zenith passage of the sun (a similar structure, with a terminal chamber carved in the rock, exists at Xochicalco, some 70 kilometers south of Mexico City (Lebeuf 1995)).
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