The Thirteen Towers constitute an ancient solar observatory

It comprises a group of 2,300-year-old structures, known as the Thirteen Towers, which are found in the Chankillo archaeological site, Peru.

The towers span the annual rising and setting arcs of the Sun, providing a solar calendar to mark special dates.

The study is published in the journal Science.

Clive Ruggles, professor of archaeoastronomy at Leicester University, UK, said: "These towers have been known to exist for a century or so. It seems extraordinary that nobody really recognised them for what they were for so long.

The towers have inset staircases

"I was gobsmacked when I saw them for the first time - the array of towers covers the entire solar arc."

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.

When viewed from the western observation point, the Sun appears to the left of the left-most tower

"For example," said Professor Ruggles, "if you were stood at the western observing point, you would see the Sun coming up in the morning, but where it would appear along the span of towers would depend on the time of the year."

"So, on the summer solstice, which is in December in Peru, you would see the Sun just to the right of the right-most tower; for the winter solstice, in June, you would see the Sun rise to the left of the left-most tower; and in-between, the Sun would move up and down the horizon."

This means the ancient civilisation could have regulated a calendar, he said, by keeping track of the number of days it took for the Sun to move from tower to tower.

Sun cults

The site where the towers are based is about four square kilometres (1.5 square miles) in size, and is believed to be a ceremonial centre that was occupied in the 4th Century BC. It is based at the coast of Peru in the Casma-Sechin River Basin and contains many buildings and plazas, as well as a fortified temple that has attracted much attention.

The authors of the paper, who include Professor Ivan Ghezzi of the National Institute of Culture, Peru, believe the population was an ancient Sun cult and the observatory was used to mark special days in their solar calendar.

The site contains a number of interesting structures

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Professor Ruggles said: "The western observing point, and to some extent, the eastern one, are very restricted - you couldn't have got more than two or three people watching from them. And all the evidence suggests that there was a formal or ceremonial approach to that point and that there were special rituals going on there.

"This implies that you have someone special - the priests perhaps - who watched the Sun rise or set, while in the plaza next door, the crowds were feasting and could see the Sun rise, but not from that special perspective.

Written records suggest the Incas were making solar observations by 1500 AD, and that their religion centred on Sun worship.

"We know that in Inca times, towers were used to observe the Sun near the solstices, which makes you speculate that there are elements of cult practice that go back a lot further," Professor Ruggles told the BBC News website.