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Observing space 2,200 years ago
Author: Clive Ruggles, Emeritus Professor of Archaeoastronomy
I have had the rare privilege of following through a once-in-a-lifetime archaeological discovery, made in 2005, to its recognition as World Heritage 16 years later. The thirteen towers of Chankillo, an extraordinary monument located in the arid coastal region of northern Peru, had been noted for decades. What was not known was that the towers represented part of a calendrical observation instrument built on a monumental scale, quite unique in the world. It was constructed over 2,200 years ago and still functions today.
I have spent a lifetime working in the specialist field of archaeoastronomy, investigating the significance of ancient buildings and monuments in relation to the sky. In 2005 I was directing another project hundreds of miles to the south, investigating markings (“geoglyphs”) and formalised pathways built across many square kilometres of desert floor in the vicinity of the famous “Nazca lines” during the early centuries AD. One of our Peruvian collaborators, Iván Ghezzi, mentioned the Chankillo site and suggested we visit and survey it together to examine its astronomical potential. I was not keen to be distracted from our investigations of a 4km-long labyrinth, itself quite unique, which I had discovered by accident over 20 years earlier, but we used four spare days at the end of our fieldwork season to make the 700km journey to the north taking with us the survey equipment.
Chankillo dates to between 250 and 200 BC, a turbulent period when the powerful “Chavín culture” had collapsed and warfare was rife. The first thing that came into sight was its “fortified temple” surrounded by three huge defensive walls, draped incongruously over the edge of the desert plateau overlooking the Casma River valley below. Prominent among the various buildings and plazas stretching away into the distance along the valley, now mostly reduced to their foundations and covered in dry sand, is a low hill with the line of 13 towers standing out conspicuously along its north-south ridge.
The thirteen towers are substantial stone-and-mortar constructions, between 2m and 6m tall and some 10m on a side. They were originally flat-topped, giving them a cuboidal shape. Although collapsed, enough remained of 51 of the 52 corners to enable their original position to be accurately determined, and that was critical for us. Iván had already excavated the area around a curious open doorway in an adjacent building that faced the towers, and discovered a scatter of votive offerings, indicating that this was a spot of considerable sacred significance. Someone standing at this entrance would have had a clear view of the towers looming over them to the east, their flat tops and the deep narrow gaps between them forming a distinctive “toothed” horizon.
It took less than two days to determine that, as viewed from this vantage, the towers coincided almost exactly with the stretch of horizon where the sun could be seen to rise at some point during the year, between its extremes at the two solstices. This was a place where just one or perhaps two people at most—presumably people of high social status such as a chief or priest—could have stood and watched sunrise against the towers. Doing this would have enabled them to determine the exact date in the solar year to within a day or two, each gap between the towers marking a progression of about ten days, slowing near the solstices at each end.
Excavations had revealed an isolated structure, sadly almost completely destroyed, on the far side of the towers. This, it turned out, would have been perfectly positioned to observe sunsets against the towers in a similar way.
We knew instantly that we had made a significant discovery. Chankillo predates the Inca Empire—whose leaders are known from early chroniclers’ accounts to have regulated agricultural and ceremonial activities by observations of sunrise or sunset against pillars constructed on the horizon—by more than 1500 years. A complete solar calendar on such a monumental scale was not only unheard of in the Americas but also in the entire ancient world.
Our paper was published in the journal Science early in 2007.
Since 2008 I have been involved in World Heritage. This came about when, as president-elect of the International Astronomical Union’s Commission on the History of Astronomy, I was asked by the IAU to help them develop a joint initiative with UNESCO aiming to encourage governments around the world to identify more potential World Heritage Sites relating to astronomy.
I ended up co-ordinating the initiative from the IAU side throughout its ten-year existence, working mainly with ICOMOS, UNESCO’s advisory body for cultural heritage, to develop guidelines for assessing the heritage value of the widest possible range of sites and artefacts relating to astronomy. We covered everything from ancient sites relating to the sky and historical observatories through to modern observatories, dark sky sites, and even heritage relating to space exploration.
I also started to advise particular countries on potential World Heritage proposals, and I was delighted to be told in 2017 that Peru was considering putting forward Chankillo, whereupon I was asked to help in developing the nomination dossier. The hoped-for result of a long assessment process came about on 27 July this year, when the UNESCO World Heritage Committee inscribed Chankillo solar observatory and ceremonial centre (“Chankillo archaeoastronomical complex”) on the World Heritage List.
The use of the word “observatory” is highly charged. Many sites around the world have been claimed to have been “ancient observatories”, but the term can be misleading because astronomical observations always had wider social purposes. In many cases it is not clear that the claimed alignments were even intentional, especially if they have been selected (by modern investigators) from many possibilities. For these reasons many so-called “ancient observatories” are highly questionable. Chankillo is not one of them.
As UNESCO puts it*, Chankillo’s solar observatory is “an outstanding example of ancient landscape timekeeping, a practice of ancient civilizations worldwide, which used visible natural or cultural features. … The astronomical facilities at Chankillo represent a masterpiece of human creative genius.”
World Heritage listing will bring many tourists to the site, of course, but should also stimulate funding to help protect the site from damage, resulting either from people’s actions or from natural causes such as earthquakes. The overriding need, as at any World Heritage site, is to preserve the attributes of its “outstanding universal value”, considered by UNESCO to be so exceptional that it transcends national boundaries and is of importance for all humanity both now and in the future. At Chankillo it is vital, for instance, to protect the fragile tower corners that are crucial to the integrity and authenticity of the functioning solar observatory.
Archaeoastronomical investigations at the site since 2007 have, amongst other things, revealed how the monumental calendar may have developed, starting with initial observations of sunrise along the jagged mountains of the distant eastern horizon. But what was the purpose of such an elaborate mechanism for marking the date precisely? There is clear evidence that large ceremonies involving music and feasting took place in the surrounding plazas. It is likely that a political or religious elite were regulating calendrical festivals, and quite possibly maintaining control over the timing of planting and harvesting. Another important factor could have been the ritualistic timing of warfare. Only a small proportion of the wider site at Chankillo has been excavated to date, and ongoing archaeological research may well produce some clearer insights. For my own part I am hoping that further archaeoastronomical work will contribute significantly to this broader picture.
* Decisions adopted during the extended 44th session of the World Heritage Committee, http://whc.unesco.org/document/188949, pp. 361–2.
About Clive Ruggles, Emeritus Professor of Archaeoastronomy
Clive is Emeritus Professor of Archaeoastronomy in the School of Archaeology and Ancient History. In 2017 he was awarded the Royal Astronomical Society’s Agnes Mary Clerke Medal for a “lifetime of distinguished work in the overlapping areas of archaeology, astronomy and the history of science”. He carries on his World Heritage work today as project director for the UNESCO–IAU Portal to the Heritage of Astronomy, and within the UK serves on the Steering Group for the Jodrell Bank Observatory, which became a World Heritage Site in 2019.