Skywatchers of Ancient Mexico is about archaeoastronomy: the study of the extent and practice of astronomy among ancient civilizations. In this case the ancient civilizations come from Mesoamerica. Aveni’s approach is interdisciplinary; he incorporates information from astronomy, astronomical history, ethnohistory, ethnography, and archaeology in an attempt to synthesize what we currently know about ancient Mesoamerican astronomy. The book rarely takes the stance of proving theories about what the past was like. Rather, it is intended as a sounding board for ideas, and the reader is frequently left to draw his own conclusions.
The text is divided into five chapters. Chapter 1 introduces the reader to the developing field of archaeoastronomy. Chapter 2 provides background information on Mesoamerican chronology and archaeology; the documentary record, especially as it pertains to the heavens; Aztec constellations; and the persistence of beliefs in Middle America today concerning solstices, equinoxes, and the zenithal sun. Chapter 3 functions as a user’s guide on positional astronomy. It is oriented to naked-eye astronomy, particularly to observers in the tropics. The movements of the sun and moon, eclipse prediction and eclipse cycles, and the planetary cycles of Mars and Venus are given extensive treatment. Various appendixes that accompany this chapter provide a helpful guide for determining the time and place of occurrence of important celestial events. Chapter 4 is devoted to the Mesoamerican calendar, perhaps the most sophisticated timekeeping system ever devised by an ancient people. Chapter 5 deals with two subjects: the alignment of Mesoamerican centers and pre-Columbian observatories. The chapter concludes with a brief discussion of archaeoastronomical achievements in North and South America.
The book’s most fascinating aspect is its demonstrations that the movements of celestial bodies can be charted very precisely with an instrumentality no more sophisticated than two sticks to mark the positions of objects on the horizon. And if observations are repeated often enough, cyclic events like eclipses, helical risings and settings of planets and stars, and the synodic periods of Venus and Mars can be predicted with great accuracy. Such periodicities have enormous implications for precise time reckoning, for they appear to have formed the basis for the ancient Mesoamerican calendar. Indeed, Aveni comes amazingly close to explaining why the ancient Mesoamericans kept two calendars: the first, the 365-day vague year; the other, the 260-day Calendar Round. One of the keys appears to be the number 260, which is a multiple both of the synodic period of Mars and three eclipse half-years, and is strikingly close to the actual appearance interval of Venus as morning and evening star. Another is that five synodic periods of Venus are almost exactly eight tropical years. A third is the so-called Metonic Cycle (i.e., the period that returns the full moon to the same date in the calendar year), which could have been used to keep track of the position of the tropical year (265.2422 days) within the vague year. When taken together, calculations of this type would give the priest-astronomer remarkable control over the passage of time.
Equally revealing is Aveni’s treatment of site alignments. Mesoamericanists have always been curious why certain sites were oriented in certain directions. The ancient city of Teotihuacón, for example, is aligned in two directions: north-south orientation, 15 degrees 28 seconds east of north; east-west orientation, 16 degrees 30 minutes south of east. Both of these alignments match baselines between sets of cross petroglyphs that are oriented to the helical rising of the Pleiades on the first day that the sun crosses the celestial zenith. The reason the two alignments do not exactly form a right angle is due to the fact that the ancient Teotihuacanos probably used two different sighting points: the Pyramid of the Sun for the north-south alignment; a locality near the Viking Group for the east-west alignment. Other Mesoamerican centers align differently, but are always linked to important, cyclic celestial events.
Skywatchers has two major drawbacks. First, there is little discussion of the role astronomical knowledge had in ancient Mexican society. For example, we are told that the 260-day period is close to the growth period for maize. The implication is that a primary role of the priestly establishment was to inform the population at large of the beginning of the agricultural season. This may be true in ceremonial terms; however, our impression is that farmers today (and during the conquest period) behave very opportunistically with respect to changing environmental conditions. Individual households always attempt to predict environmental conditions for the coming year, and this affects decisions concerning where and when to plant. It seems unlikely that the timing of these decisions coincided with priestly decrees. Thus, we really do not know why ancient Mesoamerican man was so interested in charting the movement of celestial bodies. Second, Aveni constantly alludes to the fact that astronomy among the Maya was more highly developed than among the civilizations of the Mexican highlands. To be sure, the codices from the Maya area contain a plethora of astronomical information. In contrast, those from the Mexican highlands are mainly tribute lists: an entirely different kind of document. Given the fact that calendrics and astronomically linked alignments were common to both areas, it seems likely that the practice of archaeoastronomy was equally developed in both areas. Obtaining an adequate sample of pre-Columbian documents appears to be the main problem here, not differences in the amount and kind of accumulated knowledge.
As an amateur astronomer myself, I believe that this volume would make an excellent addition to any Latin Americanist’s bookshelf. Aveni’s style is clear, and the subject matter is treated in an unbiased manner. The true value of Skywatchers is Aveni’s demonstration that archaeoastronomical achievements that appear incredible to us represent phenomena that can be accurately measured using the simplest sighting equipment. Pre-Columbian man was not subject to influences from Atlantis, Lemura, extraterrestrials, or any of the Old World civilizations. Rather, he was a keen skywatcher. If the reader appreciates this point, then Aveni’s task will have been admirably served.
