A light flashed on: Towers were a solar observatory
Archeologists have solved the mystery of the Thirteen Towers, a line of low stone structures that have spanned an arid Peruvian slope like a massive set of prehistoric teeth for 2,400 years.
The towers lined up outside the citadel at Chankillo are a massive solar observatory that marks not only the summer and winter solstices, but also the days and weeks of the year.
The evidence that they are an observatory is unequivocal, said Clive Ruggles, a professor of archeo-astronomy at the University of Leicester and one of the authors of the paper in today’s issue of the journal Science.
“It seems extraordinary that an ancient astronomical device as clear as this could have remained undiscovered for so long,” he said.
The site is not the oldest solar observatory in the New World. That honor goes to a 4,200-year-old site just north of Lima, Peru’s capital, that marks the solstices. Other ancient structures have been found that clearly have astronomical alignments.
“Unlike all the other sites, however, [Chankillo] contains alignments that cover the entire solar year,” said coauthor Ivan Ghezzi, who was a graduate student at Yale University when he did the work but is now archeological director of the National Culture Institute in Lima.
In effect, it is the oldest “full-service observatory” in the Western Hemisphere.
The finding is important because of the insight it provides on the culture of the indigenous peoples, who were ancestors of the Inca, said archeologist Clark Erickson of the University of Pennsylvania, who was not involved in the research.
“The real goal of archeology is not to find stuff, but to find out what was going on in people’s minds in the past,” he said. “The most important thing is to people the past and make it come alive, and that is what this does.”
Chankillo is a large ceremonial center covering a little more than 1.5 square miles of the Casma-Sechin River basin in the coastal Peruvian desert.
Its most notable feature is the citadel, a 900-foot-long structure built within three concentric, roughly circular walls, with gates and defensive parapets. The citadel’s resemblance to the head of an electric shaver has led some observers to refer to it facetiously as “Norelco-land.”
Archeologists have argued for more than a century over the citadel’s purpose. Many believe it is a fortress, but the lack of water inside suggests that is unlikely. The new findings support the argument that it is a ceremonial center of some sort.
To the east of the citadel is a ceremonial-civic area containing the Thirteen Towers.
Spread over a distance of nine football fields in a near-straight line, the towers range from 6 to 18 feet high and are spaced about 15 feet apart. Each has a pair of staircases, on the north and south sides, leading to the top.
Some archeologists have speculated that the towers marked lunar months, Ghezzi said, but “no one followed up on it.”
Ghezzi decided to do so in 2001. “We were there. We had extraordinary support,” he said. “We said, ‘Let’s study it while we are here.’ ”
The key was the discovery of a group of buildings about 200 yards to the west of the towers, including a building with two courtyards and a 120-foot-long exterior corridor leading to what appeared to be an observation point. The corridor, plastered and whitewashed, had no other doors and no apparent purpose other than to funnel people to the observation point.
The team found pottery, shells and other artifacts suggesting offerings associated with that opening, but not with others in the complex.
Intrigued by that finding, they looked in the same position on the east side of the towers and found the remains of a small, isolated building in the middle of a large open space. When viewed from either location, the spread of the towers provides an artificial horizon.
The two observation points are positioned, Ghezzi found, so that on the winter and summer solstices, the sun rises and sets over the towers on the opposite ends of the line, establishing the beginning and midpoint of the solar year.
Once the sun had moved away from the extreme positions of the solstices, the authors said, the various towers and gaps would have provided a means to track the progress of the sun up and down the horizon with an accuracy of two to three days.
The sun rose for just one or two days in each gap and took 10 days to proceed from one gap to the next in the center of the line, suggesting that a 10-day interval may have been a feature of the builders’ solar calendar. On the ends of the line, however, the gaps correspond to 11- or 12-day intervals.
Ruggles, one of the world’s leading authorities on archeo-astronomy, admits to having been dubious when Ghezzi approached him. “I am used to being disappointed when visiting places people claim to be ancient astronomical observatories,” he said. “The evidence all too often turns out to be unconvincing.”
Chankillo, however, “provided a complete set of horizon markers -- the Thirteen Towers -- and two unique and indisputable observation points. The fact that, as seen from these two points, the towers just span the solar rising and setting arcs provides the clearest possible indication that they were built specifically to facilitate sunrise and sunset observations throughout the seasonal year.”
Inca texts written more than a millennium later describe the use of “sun pillars” on the horizon near Cuzco to mark planting times and to regulate seasonal observances, but no traces of those pillars have been found. The Thirteen Towers clearly represent an earlier version of that technology, Ghezzi said.
“We knew that their practices of astronomy were very sophisticated and that they used buildings to mark the positions of the sun on key dates of the year, but we did not know that their practices were so old,” he said.
