Astronomical alignments in the Basin of Mexico and a fixed correlation of the prehispanic calendar with the tropical year

One can agree with Ezcurra et al. (1) that the ancient inhabitants of the Basin of Mexico used prominent mountain peaks as markers of the Sun’s positions on agriculturally significant dates. However, contending that this horizon calendar served for keeping the formal calendar in synchrony with the solar year, they resuscitate the unfounded idea that the Mexica employed an intercalation system comparable to our leap years.

Correction mechanisms that would have maintained a permanent concordance of the calendrical, 365-d year with a slightly longer tropical year have been suggested by various scholars, but none of these hypotheses can be reconciled with multiple lines of evidence, most exhaustively discussed by Prem (2). Early Spanish sources give very different dates for the beginning of the Mexica calendrical year, both because the authors collected their information in different years after the conquest (every 4 y, a native calendar date shifted back 1 d in the European calendar) and because of local differences regarding the first month of the year. Instead of presenting a critical assessment of the pertinent written sources, many of which are dubious and often contradictory, Ezcurra et al. subjectively select only a few that they interpret as supporting their view.

Their claim that the alignments matching sunrises on February 23 or 24 marked the beginning of the Mexica year is but the latest in a series of varied and unconvincing attempts to correlate the Mexica calendrical dates with those recorded by architectural alignments. A relevant and well-known fact is that the calendrical year was not synchronized throughout Mesoamerica (3); it started on different dates in different regions, whereas February 24 was marked by a large number of orientations, both in central Mexico and elsewhere (4–6), and thus could not have had any direct relationship with the calendrical year (even if we were willing to accept its fixed correlation with the tropical year). The most likely significance of this date is that the interval from October 17, which is the other sunrise date recorded by the same group of alignments (incorrectly given as October 19 by Ezcurra et al., Table 1), to February 24 is 130 d. A general characteristic of solar orientations in Mesoamerican architecture is that the corresponding dates tend to be separated by multiples of 13 or 20 d; since these were elementary periods of the formal calendrical system, the orientations and alignments to horizon prominences enabled the use of easily manageable observational calendars, thus facilitating proper scheduling of agricultural and related ritual activities. This has been demonstrated by a number of systematic studies based on analyses of large samples of alignment data (4–10). Only some of them are cited in this paper but without any attempt to confront them with the authors’ own proposals.

In sum, while the astronomical and agricultural significance of the alignments discussed by Ezcurra et al. is hardly disputable, their failure to rely on compelling evidence and to consider the results of previous archaeoastronomical studies rests credibility to the main point of their argument.