Peru's 13 ancient towers a solar observatory

For 2,400 years the Thirteen Towers standing in a line across an arid Peruvian slope have puzzled archaeologists. Now, researchers say the towers outside the citadel at Chankillo are a massive solar observatory.

    The Thirteen Towers mark the summer and winter solstices, and the days a weeks of the year, the researchers said.

    The evidence that it is an observatory cannot be disputed, said archaeo-astronomer Clive Ruggles of the University of Leicester, one of the authors of the paper in Friday'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 finding is important because of the insight it gives into the culture of the indigenous peoples, who were precursors of the Inca, said archaeologist Clark Erickson of the University of Pennsylvania, who was not involved in the research.

    "The real goal of archaeology 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 inside three concentric, roughly circular walls, with restrictive gates and defensive parapets.

    Archaeologists 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 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 in height from 6 to 18 feet and are spaced about 15 feet apart. Each tower has a pair of staircases, on the north and south sides, leading to the top.

    Some archaeologists have speculated that the towers marked lunar months, Ghezzi said, but "no one followed up on it."

    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 appears to be an observing point. The corridor, plastered and whitewashed, had no other doors and no apparent purpose other than to funnel people to the observing point.

    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 observing 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 their solar calendar. On the ends of the line, however, the gaps correspond to 11- or 12-day intervals.

    Inca texts written more than a millennium later describe the use of "sun pillars" standing on the horizon near Cuzco to mark planting times and regulate seasonal observances, but traces of those pillars have never 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.

2007-03-02