The identification of places from which astronomical observations were made in prehistory, together with evidence on the nature and context of those observations, can reveal much about the ways in which people before the advent of written records perceived, understood, and attempted to order and control the world they inhabited. Evidence of systematic observations of the changing position of the rising and setting sun along the horizon, in particular, can provide information on the development, nature and social operation of ancient calendars. Solar horizon calendars were certainly important among indigenous Americans, one of the best-known modern examples being at the Hopi village of Walpi. In pre-contact Mesoamerica, systematic studies of the orientations of sacred buildings and city plans strongly suggest the existence of horizon calendars in which special significance was attributed to certain key dates. It has been argued that these include not only the solstices, but also the
Chankillo
a 2,300-Year-Old Solar Observatory in Coastal peru
Author: Ivan Ghezzi and Clive Ruggles
From Science 315 (2007):120–124.
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7.1 Map of the Chankillo site.
dates of solar zenith passage5 and dates counted off from these at intervals significant in the intermeshing cycles of the Mesoamerican calendar round.6 In South America, accounts going back to the 16th century record various details of pre-conquest practices relating to Inca state-regulated sun worship and related cosmological beliefs.7 Various schemes of landscape timekeeping have been suggested, supported by a combination of historical evidence and analyses of the spatial disposition of sacred architecture—and in particular the system of shrines placed along lines (ceques) conceived as radiating out from the central sun temple, the Coricancha, in Cusco.8 “Sun pillars” are described by various chroniclers as having stood around the horizon from Cusco and been used to mark planting times and regulate seasonal observances,9 but all the Cusco pillars have vanished without trace and their precise location remains unknown. Here, we describe a much earlier structure in coastal Peru that seems to have been built to facilitate sunrise and sunset observations throughout the seasonal year.
The group of structures known as the Thirteen Towers is found within Chankillo, a ceremonial center in the Casma-Sechín River Basin of the coastal Peruvian desert (Fig. 7.1). Seventeen 14C dates fall between 2350–2000 calibrated years before the present (B.P.) (Fig. 7.2), and point to the beginning of occupation at the site in the fourth century BC, during the late Early Horizon period.10 The site contains multiple
7.2 Calibrated years B.P. date ranges (±SE) for samples from Chankillo, prepared by means of the program OxCal version 3.1011 with the use of Southern Hemisphere atmospheric data.12 For each sample, the first column represents the laboratory (NSF-Arizona Accelerator Mass Spectrometry Laboratory) identification number. The shaded area refers to the probability distribution of possible intersection points with the calibration curve, and the horizontal line below represents the 2-sigma calibrated age range. Five dates (AA57020 to AA57025) were sampled following dendrochronological principles from the outer sapwood rings preserved under bark in algarrobo (Prosopis sp.) lintels found still plugged into the architecture; these give a firm date for the construction of the site. The rest were obtained from the remains (including seed and fiber) of plants with short life spans. Thus, the “old wood” problem, especially troubling on the coastal desert of Peru, was minimized. CalBC, calibrated years BCE; CalAD, calibrated years CE.
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7.3 Plan of the Thirteen Towers and adjacent buildings in Chankillo. (A) Location within Peru. (B) The Thirteen Towers. (C) The external corridor and western observing point. (D) The eastern observing point.
7.4 The Thirteen Towers of Chankillo, as viewed from the fortified temple. Tower 1 is the leftmost tower in the image.
Ivan Ghezzi and Clive Ruggles
7.5 The fortified temple at Chankillo (Photo courtesy of Servicio Aerofotografico Nacional, Peru).
standing structures and plazas over approximately 4 km2 of rock outcrops and sand ramps. It is oriented south of east (azimuth 118°). Its best-known feature is a 300m-long hilltop structure built in a remote location and heavily fortified with massive walls, restricted gates, and parapets (Fig. 7.5). This famous structure has been discussed often as a fort, a redoubt, or a ceremonial center.13 However, recent research supports an alternative interpretation as a fortified temple.14 A lesser-known part of the site is a ceremonial-civic area to the east, which contains buildings, plazas, and storage facilities. The Thirteen Towers form the most outstanding feature within this area: a row of 13 cuboidal constructions placed along the ridge of a low hill (Fig. 7.3b). The towers run north-south, although Towers 11–13 are twisted around slightly towards the southwest. Seen from the buildings and plazas below this hill, on either side, the towers form an artificial toothed horizon with narrow gaps at regular intervals (Fig. 7.4).
The towers are relatively well-preserved; their corners have mostly collapsed, but enough of the original architecture survives to allow reconstruction (Fig. 7.3). They were flat-topped and rectangular to rhomboidal. Their size (75–125 m2) and height (2–6 m) vary widely. Nonetheless, they are regularly spaced: the gaps between the towers vary from 4.7 to 5.1 m. Each tower has a pair of inset staircases leading up to the summit on the north and south sides (Fig. 7.6). Most of the northern staircases
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7.6. Oblique view of Tower 1 with excavated northern staircase.
Ivan Ghezzi and Clive Ruggles
are centered along this side, although not all are aligned with the general orientation of the tower. Most of the southern staircases are often offset toward the east. The staircases are narrow (1.3–1.5 m wide), but because the heights of the towers vary, they are of different lengths (1.3–5.2 m). Most of the tower summits are well preserved; no artifacts remain on these surfaces, though it is clear they were foci of activity.
A group of enclosures is found 200 m to the west of the towers (Fig. 7.3). The southernmost enclosure contains a building composed of two courtyards. The southeast courtyard is 53.6 m long and 36.5 m wide, and is well-preserved. Running along its southern side is a unique construction: a 40 m-long exterior corridor (Fig. 7.3c). The corridor, like the rest of the building, was carefully constructed, plastered, and painted white; however it never led into it. Instead, it connected a doorway on the northwest side, to which access was restricted by a blocking wall, with an opening on the southeast side that directly faced the towers 235 m away. The southeast opening, unlike every other doorway at Chankillo, did not have the typical barholds, or small niches where a pin was firmly tied into the stone masonry and presumably used to attach a wooden door.15 We infer that the purpose of the corridor was to orchestrate movement from its restricted entryway to a doorless opening directly facing the towers. Considering the original height of the corridor walls, estimated at roughly 2.2 m, only when the opening was reached would there have been an unobstructed view of the full row of towers. Surrounding the opening, at floor level, archaeological excavations have revealed offerings of pottery, shell, and lithics. No other offerings were found associated with openings in excavations elsewhere at the site.16 This suggests that significant elements of ritual were involved in the process of passing through the corridor and standing at the end of it to observe the towers. Consequently, we term this opening the “western observing point.”
To the east of the towers (Fig. 7.3) is a large area (1.4 km2) with several buildings, including an impressive complex of interconnected patios and rooms, chicha (corn beer) storage facilities, and a large plaza (0.16 km2). In several places within the plaza there were surface offerings of ceramic panpipes and thorny oyster (Spondylus princeps sp.), and middens near the plaza contained remains of serving vessels, more ceramic panpipes, and abundant maize remains. This whole area was probably a setting for large ceremonial feasts.
From several locations around this ceremonial area, the Thirteen Towers command the landscape and could be used as solar horizon markers, but one isolated building is of particular interest (Fig. 7.3d). It is a small, isolated building in the middle of a large, open space. Its position in relation to the Thirteen Towers is almost an exact mirror of the western observing point: the two lie almost exactly on an east-west line, are at the same elevation, and are at roughly the same distance from the towers. When viewed from inside this building, the spread of the towers forms an artificial horizon as well.
Only an incomplete outline of a rectangular room, 6 m wide, is preserved. Like the corridor leading to the western observing point on the opposite side of the towers, this room had a doorway—in this case on the southeast side—that was restricted by a small blocking wall. We hypothesize that this structure was the eastern observing point, but the exact position cannot be known with the same certainty as the western observing point.
We determined the locations of the two observing points together with the corners of each tower using hand-held differential GPS equipment. This enabled each point on the false horizon formed by the towers, as viewed from each observing point in turn, to be defined in terms of its azimuth, altitude, and (astronomical) declination (Tables 7.1 and 7.2, for more detail, see Tables 7.3 and 7.4). Independent compassclinometer determinations of azimuths and altitudes, calibrated using a direct observation of sunrise against the towers, provided consistency checks. By “altitude,” we mean the vertical angle between a viewed point and the horizontal plane through the observer, “elevation” being the height of a location above sea level.17
Declinations of +23.75° and –23.75° correspond to the center of the sun at the extreme positions of sunrise and sunset in 300 BC, at the June and December solstices respectively, with the sun’s disc extending between +23.5° and +24.0° (June) and between –24.0° and –23.5° (December).18 Intermediate declinations correspond to sunrise and sunset on other dates.
As viewed from the two observing points, the spread of the towers along the horizon corresponds remarkably closely to the range of movement of the rising and setting positions of the sun over the year. This in itself argues strongly that the towers were used for solar observation. From the western observing point, the southern slopes of Cerro Mucho Malo, at a distance of 3 km, meet the nearer horizon (formed by the nearby hill on which the towers are constructed) just to the left of the northernmost tower, Tower 1, providing a thirteenth “gap” of similar width to those between each pair of adjacent towers down the line.
From the eastern observing point, the southernmost tower, Tower 13, would not have been visible at all, and the top of Tower 12 would only just have been visible (it is only partially visible now in its ruinous condition). From here, the December solstice sun would have been seen to set behind the left side of the southernmost visible tower, Tower 12, while the June solstice sun set directly to the right of the northernmost tower, Tower 1 (Fig. 7.7). In either case, once the sun had begun to move significantly away from either of its extreme rising positions a few days after each solstice, the various towers and gaps would have provided a means to track the progress of the sun up and down the horizon to within an accuracy of two or three days.
If we accept that the towers were used as foresights for solar observations, then does their disposition suggest anything about the way the year might have been broken down? The flat tops of the towers originally formed their own smooth, “false” horizon, their varying heights compensating to some extent for the slope of the hill on which they were built. This false horizon was broken at intervals by deep, narrow cuts formed by the gaps between the towers. When viewed from the western observing point, the sun rose for just one or two days in each gap. One possibility, then, is that critical sunrises were observed in the gaps. However, the regularity of the gaps argues against this, suggesting instead that the year was divided into regular intervals. The
Table 7.1. The annual movement of sunrise against the line of towers as viewed from the western observing point.
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This table lists the declination of the center top of each tower, and the gap between each pair of towers. The following columns show the dates in the year (using the Gregorian calendar, taking the June solstice as June 21) when the sun would have risen at the point in question, and the intervals (numbers of days) between sunrises in successive gaps. The quoted dates and the intervals are only accurate to within ± 1 day at best, with larger errors possible near the solstices, where the daily change in the sunrise position is extremely small.
Table 7.2. The annual movement of sunset against the line of towers as viewed from the eastern observing point.
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The details are similar to Table 7.1. Note that Tower 13 was not visible, and that no gaps were visible between Towers 12 and 11 or between Towers 11 and 10.
sunrises in the gaps between the central towers, Towers 3 to 11, were all separated by time intervals of (or close to) 10 days, implying that a 10-day interval may have been a feature of the solar calendar. However, the time intervals are longer between the outer towers in the line, where the sunrise moves along more slowly. Furthermore, the situation is different from the eastern observing point (Fig. 7.8), since no gaps would have been visible between the southernmost towers in the line as far as Tower 10 (and
7.7 The Thirteen Towers as viewed from the western observing point, annotated with the positions of sunrise at the solstices, equinoxes, and the dates of zenith and antizenith passage in c. 300 BCE. Tower 1 is the leftmost tower in the image.
7.8 The Thirteen Towers as viewed from the eastern observing point, annotated with the positions of sunset at the solstices, equinoxes, and the dates of zenith and antizenith passage in c. 300 BCE. Tower 1 is the rightmost tower in the image.
7.9 June solstice sunrise between Cerro Mucho Malo and Tower 1, as viewed from the western observing point today. The position of the June solstitial sunrise has shifted rightwards by about 0.3° since 300 BC.
possibly 9), and the remaining gaps correspond to time intervals between sunsets of 11 or 12 days (Table 7.2).
From the eastern observing point, the December solstice sun set into the left side of the leftmost visible tower whereas the June solstice sun set into the right side of the rightmost tower. From the western observing point, the December solstice sun rose up from the top of the rightmost tower while the June solstice sun rose a little way up the slopes of Cerro Mucho Malo (Fig. 7.9). There is an evident symmetry here also, suggesting that this natural hill was perceived as the leftmost “tower” in this profile. Midwinter would have been the one time of year when the sun was seen to emerge from a natural hill rather than a human construction.
Equinoctial sunrise (declination 0.0°) occurred in the central gap directly between Towers 6 and 7. If Cerro Mucho Malo is included, so that there are thirteen gaps, then this is the central one. In the other direction, equinoctial sunset occurred just to the right of this same gap, which seen from the east is the central gap within the twelve visible towers. However, the applicability of the concept of the equinox outside a Western conceptual framework is highly questionable.19 If, as here, there is clear evidence that a mechanism existed to help count off the days, then the mid-days between the solstices (the “temporal equinoxes” or “Thom equinoxes”) are more likely to have been significant. In 300 BC, the sun’s declination on these days was between +0.6° and +1.0°, and there is no evidence that these days were specially marked.
Table 7.3. The annual movement of sunrise against the line of towers as viewed from the western observing point.
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The data given for the sides and center-top of each tower consist of the (true) azimuth, altitude, and declination, quoted to the nearest 0.1°. The following columns show the dates in the year (using the Gregorian calendar, taking the June solstice as June 21) when the sun would have risen behind the point in question, and the dates when it would have risen in the gaps between each pair of towers. The quoted dates, and the quoted intervals (numbers of days) between them, are only accurate to within ± 1 day at best, with larger errors possible near the solstices, where the daily change in the sunrise position is extremely small.
Table 7.4. The annual movement of sunset against the line of towers as viewed from the eastern observing point.
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The details are similar to Table 7.3. Note that Tower 13 was not visible, and that no gaps were visible between Towers 12 and 11 or between Towers 11 and 10.
Ivan Ghezzi and Clive Ruggles
7.10 (A) Warrior ceramic figurine. Weapon types found at Chankillo: (B) spear; (C), (D) and (E) clubs; (F) spear-thrower; (G) darts; (H) sling; (I) shield.
A variety of evidence suggests that the date of solar zenith passage was significant to early cultures in the American tropics in general and in the Andes in particular.20 It has also been suggested that the dates of solar antizenith passage might have been of significance in Inca Cusco,21 although this idea has been debated.22 However, there is nothing in the pattern of disposition of the towers to suggest that it was deliberately preconceived in relation to sunrise or sunset on these dates. Only zenith passage sunset falls close to (and even then, not exactly within) a gap between two towers.
Astronomical “explanations” can be fitted notoriously easily to preexisting alignments. Repeated instances of solar and lunar alignments can provide strong evidence of intentionality, as among many local groups of later prehistoric tombs and temples in Britain, Ireland and Europe.23 However, at a unique site there is always a danger of circular argument if the judgment of what might have been significant to people in the past is made solely on the basis of the alignment evidence itself. Fortuitous stellar alignments are particularly likely, given the number of stars in the sky and the fact that their positions change steadily over the centuries owing to precession. The Chankillo towers, on the other hand, just span (to within a couple of degrees) the solar rising and setting arcs as seen from two observing points, each clearly defined by a unique structure with no other apparent purpose. Thus we are not selecting putative astronomical targets from innumerable possibilities but seeing direct indications of all four solstitial rising and setting points—astronomical “targets” whose broad significance across cultures is self-evident and widely attested.
It is uncontroversial to postulate direct observations of the annual movement of the rising or setting sun along the horizon for the purposes of regulating seasonal events such as religious festivals, or for maintaining a seasonal calendar. Nonetheless, evaluating the nature of the observations made and the social and ritual context within which they operated and derived their relevance is not simple. This point is well illustrated by recent debates concerning the function of the so-called E-group structures in the Mayan heartlands of the Peten, Guatemala.24 In the case of the Thirteen Towers and nearby plazas, we can infer, they provided a setting for people participating in public rituals and feasts directly linked to the observation and interpretation of the seasonal passage of the sun. By contrast, the observing points themselves appear to have been highly restricted. Individuals with the status to access them and conduct ceremonies would have had the power to regulate time, ideology, and the rituals that bound this society together. Additionally, the excavations at Chankillo have uncovered ceramic warrior figurines holding a great variety of offensive (and defensive) weapons (Fig. 7.10).25 The figurines wear signs of distinction, such as headdresses, shirts, and especially neck, chest, and nose ornaments. The artistic representation of these warriors, holding specialized weapons and wearing the symbols of their high status, indicates the possible rise of a class of war leaders and the centralization of power and authority in the hands of a few. Thus, sun worship and related cosmological beliefs at Chankillo could have helped to legitimize the authority of an elite, just as it did within the Inca empire two millennia later. And this, in its turn, implies that the towers were not a simple instrument for solar observation but the monumental expression of existing—and therefore by implication even older—knowledge.
There is increasing evidence that the sun cult, which as the official cult of the Inca empire, regulated calendrical ceremonies and supported the established social hierarchy, had precursors. For example, historically attested sunrise ceremonies at a sanctuary on the Island of the Sun in Lake Titicaca,26 surrounding a crag regarded as the origin place of the sun, almost certainly had pre-Incaic roots.27 Given the similarity between the solar observation device at Chankillo and the Cusco pillars documented some two millennia later,28 it seems likely that similar practices were common within many of the great states that developed in the Andes prior to, as well as including, the Inca empire.
Notes
We thank the numerous archaeologists and volunteers who participated in the Chankillo project, and especially J. L. Pino. We thank Yale University, Pontificia Universidad Católica del Peru, National Science Foundation, Wenner-Gren Foundation, The Field Museum, Schwerin Foundation, and Earthwatch Institute for support. R. Towner and K. Anchukaitis were instrumental in securing five samples for dendrochronological dating. The NSF funded all AMS radiocarbon dates. We thank the Asociación Cultural Peruano Británica in Lima, Peru, for logistical and financial support.
Homenaje a Maria Rostworowski, ed. R. Varon and J. Flores (Lima: IEP, 1997), 567–590.
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