Puzzle solved: Ancient towers an 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 last week's 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, a graduate student at Yale University when he did the work but 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.