Chankillo, a fortified hilltop in the midst of ancient Peruvian ruins, has long baffled scientists. Some believed it to be a fort, others a ceremonial site, but no one could explain the purpose of the 300-meter line of 13 towers positioned along a low-lying ridge in the Andes Mountains.

The mystery has been solved by Ivan Ghezzi GRD ’07 and Clive Ruggles of the University of Leicester in England, who published the answer in the March 2 issue of Science magazine. Chankillo was a solar observatory built in the 4th century B.C., and it is the oldest ever discovered, predating the earliest-known forays into astronomy by the Mayans by 500 years.

Ghezzi, also the Director of Archaeology of the National Institute of Culture in Lima, visited Chankillo to research his Yale thesis on ancient warfare in the region. Although there had been speculation among archaeologists about the astronomical relevance of the 13 towers, Ghezzi was the first to begin to explore the notion in 2001.

Within only a few hours of field research, Ghezzi’s team established that one tower aligned with the position of the sun at the June solstice and another with the position at the December solstice. After further research into the function of the towers, Ghezzi had enough evidence to call in Ruggles, an authority on archaeoastronomy.

Ruggles said he gets two to three e-mails a day from people claiming to have found sites with links to ancient astrology.

“I am used to being disappointed when visiting places people claim to be ancient astronomical observatories,” Ruggles said in a press release. “Since everything must point somewhere and there are a great many promising astronomical targets, the evidence … turns out all too often to be completely unconvincing.”

“Chankillo, on the other hand, provided a complete set of horizon markers — the Thirteen Towers — and two unique and indisputable observation points,” he added.

Ghezzi’s finding is “tremendous,” said Alane Alchorn, the science editor of the journal Archaeoastronomy.

“Research of this magnitude in the field hasn’t been seen in 20 to 25 years,” she said. “It pushes back the parameters for calculated predictive astronomy hundreds of years.”

Richard Burger, chair of Archaeological Studies program and Ghezzi’s graduate adviser, said the findings are unusually convincing and will cause many in the field to reevaluate their own research.

“This site has been known for a hundred years,” Burger said. “People will go back to many sites they know and wonder if they missed a dimension — do other sites have towers?”

Ghezzi’s findings were based on the discovery of two man-made observation points about 200 meters from the site. Although one of the observation points has largely deteriorated, the Western viewpoint remains intact and is found at the end of a long windowless corridor adorned with ceremonial figures. The viewpoints make the significance of the towers’ positioning clear — on the summer and winter solstices the sun rises and sets over the line of towers on opposing ends, thus determining the beginning and middle of the solar year.

“This is evidence that before the Incas and the sun-centered cult, you have people in a different area very carefully clocking solar cycles,” Alchorn said. “It would have taken 20 to 25 generations of paying attention to the position of the sun to construct the towers and the observatory.”

The importance of the solar year to this specific culture is not known, but the sun’s cycle played an important role in many civilizations’ agriculture, determining the proper time to plant crops. Ceremonies and judicial decisions were also often linked to agricultural and solar cycles. The paper’s authors also note that access to the observation point appears to have been highly restricted, suggesting, as in the Inca Empire, sun worship was used to legitimize the authority of an elite class.

Alchorn said the archaeoastronomical community is pleased that the discovery has brought increased attention to the discipline.

“We tend to think of science as atoms,” Alchorn said. “This is science integrated in everyday life in a way we lack today. People lived tied to astronomy.”