Archeologists from Yale and the University of Leicester have identified an ancient solar observatory at Chankillo, Peru as the oldest in the Americas with alignments covering the entire solar year. Recorded accounts from the 16th century CE detail practices of state-regulated sun worship during Inca times, and related social and cosmological beliefs. These speak of towers being used to mark the rising or setting position of the sun at certain times in the year, but no trace of the towers has ever been found. At Chankillo, not only were there towers marking the sun's position throughout the year, but they remain in place, and the site was constructed much earlier – in approximately the 4th century BCE.
     "Archaeological research in Peru is constantly pushing back the origins of civilization in the Americas," said Ivan Ghezzi, a graduate student in the department of Anthropology at Yale University and lead author of the study. "In this case, the 2,300 year old solar observatory at Chankillo is the earliest such structure identified and unlike all other sites contains alignments that cover the entire solar year."
     Chankillo is a large ceremonial center covering several square kilometers in the costal Peruvian desert. It was better known in the past for a heavily fortified hilltop structure with massive walls, restricted gates, and parapets. For many years, there has been a controversy as to whether this part of Chankillo was a fort or a ceremonial center. But the purpose of a 300meter long line of Thirteen Towers lying along a small hill nearby had remained a mystery. The new evidence now identifies it as a solar observatory.
     The Thirteen Towers of Chankillo run from north to south along the ridge of a low hill within the site; they are relatively well-preserved and each has a pair of inset staircases leading to the summit. The rectangular structures, between 75 and 125 square metres (807-1,345 sq ft) in size, are regularly spaced - forming a "toothed" horizon with narrow gaps at regular intervals. About 230m (750ft) to the east and west are what scientists believe to be two observation points. From these vantages, the 300m- (1,000ft-) long spread of the towers along the horizon corresponds very closely to the rising and setting positions of the Sun over the year. At the end of a 131-foot-long corridor in the building to the west of the towers, the researchers found pottery, shells, and stone artifacts in an area possibly for commoners who participated in rituals linked to solar observations. The current report offers strong evidence for an additional use of the site at Chankillo — as a solar observatory.
     Ghezzi was originally working at the site as a Yale graduate student conducting thesis work on ancient warfare in the region, with a focus on the fortress at the site. Noting the configuration of 13 monuments, in 2001, Ghezzi wondered about a proposed relationship to astronomy. To his great surprise, within hours they had measurements indicating that one tower aligned with the June solstice and another with the December solstice. But, it took several years of fieldwork to date the structures and demonstrate the intentionality of the alignments.
     In 2005, Ghezzi connected with co-author Clive Ruggles, a leading British authority on archeoastronomy. Ruggles was immediately impressed with the monument structures. "I am used to being disappointed when visiting places people claim to be ancient astronomical observatories." said Ruggles. "Since everything must point somewhere and there are a great many promising astronomical targets, the evidence — when you look at it objectively — 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," Ruggles said.
     What they found at Chankillo was much more than the archival records had indicated. "Chankillo reflects well-developed astronomical principles, which suggests the original forms of astronomy must be quite older," said Ghezzi, who is also the is Director of Archaeology of the National Institute of Culture in Lima, Peru. The researchers also knew that Inca astronomical practices in much later times were intimately linked to the political operations of the Inca king, who considered himself an offspring of the sun. Finding this observatory revealed a much older precursor where calendrical observances may well have helped to support the social and political hierarchy. They suggest that this is the earliest unequivocal evidence, not only in the Andes but in all the Americas, of a monument built to track the movement of the sun throughout the year as part of a cultural landscape.
     "Chankillo is one of the most exciting archaeoastronomical sites I have come across," said Ruggles. "It seems extraordinary that an ancient astronomical device as clear as this could have remained undiscovered for so long."

Sources: EurekAlert!, BBC News, Yahoo! News (1 March 2007)