Horizon Calendars of Central Mexico

Since the emergence of archaeoastronomy in the 1970s, there have been numerous studies of the orientations of temple-pyramids and other constructions in the central Mexican highlands and throughout Mesoamerica. Particular attention has been paid to the Valley of Mexico, in which peoples had dwelt for millennia before the Aztec capital, Tenochtitlan, was eventually founded at what is now the center of Mexico City. Numerous instances have been identified of orientation upon sunrise at the solstices and on the day of solar zenith passage, as well as on other calendrically significant dates.

It has also been proposed that horizon calendars were developed by direct observation of the movement of sunrise on successive days along mountainous horizons full of natural foresights and reference points. Some researchers have argued that surprisingly many sunrise alignments correspond to dates separated from the solstices by intervals of 20, 52, 65, or 73 days— numbers that are also encapsulated in the intermeshing cycles of the Mesoamerican Calendar Round. Among the independent strands of evidence in support of this idea is the fact that the first appearance of the noonday sun in the Xochicalco zenith tube occurs exactly fifty-two days before the June solstice, and the final appearance fifty-two days after. These successive fifty-two-day intervals are particularly intriguing because the interval between them (at Xochicalco, the length of the period of darkness before the sun starts to appear at noon again) is more or less 260 days, the length of the calendrical cycle known as the tonalpohualli.

A more controversial suggestion is that horizon calendars were tied to concepts of space, with intervals in time corresponding to horizon sunrise positions separated by multiples of an angular unit of around 4.5 degrees. However it is more likely that the true significance of dates encapsulated in intentional sunrise alignments had to do with a much richer set of perceived connections—between temples, the perceived dwelling places of the gods, the correct place and timing of tributes to ensure seasonal renewal, and so on. In other words, the horizon calendars only made sense in the context of complex schemes of sacred geography.

One idea is that horizon calendars might have been developed in the Valley of Mexico at a very early date and may have helped stimulate the development of the Mesoamerican Calendar Round that is well known from his torical and documentary evidence. One of the main proponents of this idea is the Austrian-born anthropologist Johanna Broda, who argues that an early horizon calendar existed at Cuicuilco, a large Preclassic temple-pyramid dating from the first millennium B.C.E. whose ruins are now lost within the suburbs of Mexico City. This temple, argues Broda, was built on an observation point where some of the natural features of the eastern horizon happened to mark—and others came to define—significant calendrical dates.

There are three strands to the argument. First, the main peaks on the eastern horizon correlate particularly well with sunrise on calendar dates that were significant in the later Mesoamerican calendar and some of which continue to be significant to the present day. Second, there are coincidences between these sunrise dates and the dates of the calendrical rituals performed at shrines erected on the slopes of some of the relevant mountains. And finally, the visual lines that connect Cuicuilco with the mountain peaks on the eastern horizon pass through a number of historically significant settlements, sacred mountains, and shrines, implying a network of sacred lines in the landscape that remained significant, and continued to be respected, over a long period. Broda's scheme has since been reassessed and modified by the Slovenian archaeoastronomer Ivan Sprajc, but he has confirmed the idea that calendrically significant intervals were marked at Cuicuilco.

This type of investigation raises a number of methodological questions that are more familiar from "green" archaeoastronomy and reassessments of British ley lines. Yet it rests on much firmer ground because of the abundance of historical evidence concerning sacred geography in ancient Mesoamerica.

See also:

"Green" Archaeoastronomy; Ley Lines; Methodology; Sacred Geographies.

Aztec Sacred Geography; Mesoamerican Calendar Round; Zenith Tubes.

References and further reading

Iwaniszewski, Stanislaw, Arnold Lebeuf, Andrzej Wiercinski and Mariusz Ziólkowski, eds. Time and Astronomy at the Meeting of Two Worlds, 497-512. Warsaw: Centrum Studiów Latynoamerykanskich, 1994.

Romano, Giuliano, and Gustavo Traversari, eds. Colloquio Internazionale Archeologia e Astronomia, 15-22, 123-129. Rome: Giorgio Bretschnei-der Editore, 1991.

Ruggles, Clive, and Nicholas Saunders, eds. Astronomies and Cultures, 270-285. Niwot, CO: University Press of Colorado, 1993.

Sprajc, Ivan. "Astronomical Alignments at Teotihuacan, Mexico." Latin American Antiquity, 11 (2000), 403-415.

-. Orientaciones Astronómicas en la Arquitectura Prehispánica del

Centro de México, 172f, 201f, 258f. Mexico City: Instituto Nacional de Antropología e Historia (Colección Científica 427), 2001. [In Spanish.]

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