ON THE winter solstice in 2017, a team of researchers waited in the pre-dawn chill of the Atacama desert. Before them stood two square piles of stones, each about 1.2 metres (four feet) high. A row of three other cairns stretched out 500 metres to the east. This line of saywas—roughly, “markers” in Quechua, an indigenous Andean language—intersected diagonally with an ancient path, part of a road network built five centuries ago by the Incas. The sun rose directly behind the closest columns, appearing to rest briefly atop them.

“It was an extremely moving experience,” says Cecilia Sanhueza, a historian at Chile’s Pre-Columbian Art Museum in Santiago. Her findings were made public last month. The alignment of the stones with the sun’s rise supported her thesis that they were not just milestones. At least some of northern Chile’s saywas had the “astronomical function” of prefiguring the sun’s appearance. They are a southern-hemisphere Stonehenge.

The pillars are a visible link to Inti, the sun god, who was thought to “sit” on saywas at solstices. Their arrangement was a way of “sacralising the political presence of the Inca”, whose empire ruled northern and central Chile from about 1470 to 1530, says Dr Sanhueza.

She formed her initial theories from her study of 16th-century Quechua-Spanish dictionaries, the drawings of Felipe Guaman Poma de Ayala, a Quechua nobleman who wrote and illustrated a 17th-century treatise on colonial Peru, and a chronicle by Martín de Murúa, a Basque friar.

To test her ideas, Dr Sanhueza approached the Atacama Large Millimetre Array, an observatory in the Andes mountains, around 150km (90 miles) from the saywas. Simulations by Sergio Martín and Juan Cortés, astronomers at the observatory, supported the thesis that some rows are aligned with sunrises on important dates. That spared Dr Sanhueza the trouble of testing in person the function of each set of saywas.

But she and her colleagues then spent days and nights battling altitude sickness and the cold to study the environment for additional clues to the purpose of the saywas. Jimena Cruz, an indigenous Atacameña archaeologist, interviewed retired llama herders to learn more about the cultural significance of the pathways. She suggested observing one set of saywas on August 1st, a day of veneration of the earth goddess Pachamama. Sure enough, the rising sun aligned with the pillars. Ms Cruz also recruited local volunteers to help preserve the saywas.

There are more to be studied, Dr Sanhueza thinks. The investigators hope the example they set will encourage more collaboration between archaeologists, astronomers and locals, and remind the rest of the country that it has a rich indigenous heritage.