For a long time nobody had any ideas about why exactly were the thirteen towers built in the arid Peruvian slope over 2,400 years ago.

Now archaeologists have said that the towers built near the citadel at Chankillo constituted an ancient solar observatory that marked summer and winter solstices apart from the days and weeks of the year. The purpose of the towers has been argued for over a century without reaching any satisfactory reason.

Built as a ceremonial center, Chankillo covers 1.5 square miles of the Casma-Sechin River basin in the Peruvian desert. However the most striking piece of the structure is the 300-yard-long line of towers that jut out of the ground.

Ivan Ghezzi, the archeological director of the National Institute for Culture in Peru, said that the discovery of a group of buildings some 200 yards away from the towers led the researchers to investigate the structure, which included two courtyards and a 120-foot-long exterior corridor.

The corridor did not have any other doors and led directly to a observation point. On investigating on the other side, the researchers found remains of a similar building.

Ghezzi said that the two points were positioned in such a way that during the winter and summer solstices, the sun rose and set over the towers on the opposite ends of the line. He added that this marked the beginning and midpoint of the solar year.

The evidence was published in Friday's issue of the journal Science. Clive Ruggles, an archaeo-astronomer at the University of Leicester and one of the authors of the report said, "It seems extraordinary that an ancient astronomical device as clear as this could have remained undiscovered for so long".