The many huge earthworks or mounds that remain visible in the Mississippi and Ohio valleys form a conspicuous testimonial to the technical achievements of indigenous North Americans before the arrival of European settlers. Inevitably, a number of them have attracted interest in potential astronomical alignments, and it is scarcely surprising that this includes the great Mississippian site of Cahokia.

The Mississippian culture flourished from about c.e. 1000 to 1400 in the central Mississippi valley, where the fertile floodplains were ideal for growing maize and other staple crops. Cahokia was not only the main economic and political focus of this culture but also the largest pre-Columbian town—worthy in fact of being called a city—north of Mexico. At the height of its development it covered over fifteen square kilometers (six square miles) and had an estimated population of more than twenty thousand. The city itself was laid out on a roughly cardinally aligned grid. At its heart was a ceremonial center containing over a hundred earthworks—temple or house platforms together with burial mounds. Most of this “mound center” is now preserved in Cahokia Mounds State Historic Site near East St. Louis.

The rectangular, flat-topped earthwork known as Monks’ Mound is at the center of the ceremonial grid. It is the largest earthen mound in the whole of the Americas, some thirty meters (a hundred feet) high and covering around seven hectares (seventeen acres). But it was the discovery in the early 1960s of rings of postholes marking the sites of five timber circles to the west of Monks’ Mound that triggered intense astronomical interest in the site. By analogy to Woodhenge—a set of concentric, oval-shaped timber rings found near Stonehenge in England—the discoverers named them woodhenges, although it is dangerous to draw close similarities between superficially similar sites from completely different cultural contexts. The five original woodhenges at Cahokia, identified by extrapolating from strings of postholes forming circular arcs running through excavated areas, appear to have had diameters ranging from thirty-seven meters (120 feet) to seventy-one meters (233 feet). They were good approximations to true circles, with evenly spaced posts. Most remarkably, initial estimates of the numbers of posts in the five circles—twenty-four, thirty-six, forty-eight, sixty, and seventy-two—implied that numerology was vitally important, and that strict design principles were operating. Four of the five posthole circles overlap, suggesting that they represent successive constructions fulfilling a particular purpose. Since this time, a number of other postholes and posthole structures have been discovered at Cahokia.

The original suggestion made by archaeologist Warren Wittry in the early 1960s was that the various circles of timber posts functioned as devices for observing the rising position of the sun at the solstices and equinoxes, perhaps for the purpose of regulating an agricultural or ceremonial calendar. However, this involved the totally arbitrary selection of data: for instance, three posts—indistinguishable in any other way from all the remaining posts in one of the circles—were identified as putative solar foresights as viewed from a central post. Other corroborating evidence presented at the time—for instance, relating to the orientation of the ramps used in setting each relevant timber post in its place—has proved totally fallacious, as have various further astronomical speculations.

An obvious question to have asked at the outset was: why, if sunrise observation was the primary purpose, was a whole circle of posts needed? Indeed, circles of this size—unless supplemented by the use of distant horizon foresights—cannot define the relevant alignments at all precisely. It is possible that the circles had a cosmological function or meaning, but the publicity given to the unfounded astronomical speculations has undoubtedly hampered serious consideration of this issue.

Instead, Cahokia stands as an unfortunate example of bad practice—uncritical spotting of alignments upon preconceived astronomical targets undertaken without due consideration to the broader interpretative context.

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