Scientists have discovered the oldest solar observatory in the Americas and, in the process, may have solved a centuries-old puzzle about the purpose of an ancient stone fort on a remote hilltop in Peru.
The researchers have shown that an enigmatic wall of 13 stone towers within the Chankillo complex, a 2,300-year-old ruin nearly 250 miles north of Lima, worked as a solar calendar to monitor the winter and summer solstices.
They believe that the solar observatory proves the existence of a sophisticated Sun cult in the region more than 1,000 years before the Inca civilisation built its famous Sun temple in the Peruvian mountain city of Cusco, prior to the Spanish conquest.
Ivan Ghezzi of the Pontificia Universiadad Catolica del Peru in Lima and Clive Ruggles of Leicester University have found that the line of 13 towers at Chankillo can be used to precisely observe the Sun as it rises and sets at different positions along the horizon throughout the year.
Historical accounts suggest that the Inca Sun pillars at Cusco - which have vanished without trace - were used until the 16th century AD to mark planting times of crops and to observe seasonal ceremonies, Ghezzi and Ruggles say in their study published today in the journal Science.
They believe that the discovery means that the massive Chankillo complex - dated to the 4th century BC - must have played an important role in the ceremonial rituals associated with the annual cycles of the Sun.
Archaeologists have puzzled over the purpose of Chankillo since it was first discovered in the 19th century. They suggested it may have been used as a fort, a temple or even a setting for ceremonial battles.
One of the biggest mysteries of Chankillo was the purpose of a low ridge composed of 16 relatively small stone towers which together formed an artificial toothed horizon for no apparent reason.
However, Ghezzi and Ruggles show that the gaps formed between the towers match the annual rising and setting arcs of the Sun while it dips below the horizon during the winter and summer solstices.
The line of towers, which range in height from about 6ft to 20ft, were built along a north-south axis and can be viewed full-on from two other stone positions, one to the east and one to the west of the ridge.
"Viewed from the two observing points, the spread of the towers along the horizon corresponds very closely to the range of movement of the rising and setting positions of the sun over the year," say Ghezzi and Ruggles in their study.
"This in itself argues strongly that the towers were used for solar observation," they say.
In their study, the scientists demonstrated that the setting sun at the winter solstice can be viewed from the eastern observation point as it falls to the left side of the southernmost tower.
Meanwhile, from the western observing point, the same midwinter sun the following morning could be viewed rising from the last tower on the right of the observer. During the course of the year, the setting and rising sun moves through the different "teeth" of the artificial horizon until finally it reaches its next furthermost point at the summer solstice in June - when it can be observed rising and setting beyond the last tower to the north.
"The towers are relatively well preserved; their corners have mostly collapsed, but enough of the original architecture survives to allow a reconstruction," the researchers say.
The towers are regularly spaced and each has a pair of stone staircases leading up to the summit, one on the north and one on the south side.
"Most of the tower summits are well preserved; no artifacts remain on these surfaces, though it is clear from the staircases that the summits were the foci of activity," they say.
Archaeologists have found evidence to suggest that the ceremonial practices took place at the two observing positions. The western point has offerings of pottery, shells and lithic artifacts whereas the eastern site was probably a site of ceremonial feasting, the scientists said.
The gaps between the towers may have been used to mark out the days of a solar calendar. For instance, the sunrises between the gaps in the central towers are separated by a time interval of 10 days, implying that a 10-day "week" may have been important in the solar calendar.
"Once the Sun had begun to move appreciably away from either of its extreme 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," say Ghezzi and Ruggles in their research.
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Ancient towers in Peru
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