Sao Paulo, Brazil ? A grouping of granite blocks along a grassy Amazon hilltop may be the vestiges of a centuries-old astronomical observatory – a find archaeologists say indicates early rainforest inhabitants were more sophisticated than previously believed.

The 127 blocks, some as high as 9 feet tall, are spaced at regular intervals around the hill, like a crown 100 feet in diameter.

On the shortest day of the year – Dec. 21 – the shadow of one of the blocks, which is set at an angle, disappears.

“It is this block’s alignment with the winter solstice that leads us to believe the site was once an astronomical observatory,” said Mariana Petry Cabral, an archaeologist at the Amapa State Scientific and Technical Research Institute. “We may be also looking at the remnants of a sophisticated culture.”

Anthropologists have long known that local indigenous populations were acute observers of the stars and sun. But the discovery of a physical structure that appears to incorporate this knowledge suggests pre-Columbian Indians in the Amazon rainforest may have been more sophisticated than previously suspected.

“Transforming this kind of knowledge into a monument; the transformation of something ephemeral into something concrete, could indicate the existence of a larger population and of a more complex social organization,” Cabral said.

A grouping of 127 granite blocks along a grassy Amazon hilltop near Calcoene, Brazil, may be the vestiges of South America's oldest astronomical observatory, according to archaeologists. The blocks, some standing as high as 9 feet, are spaced at regular intervals around a hill, in a ring some 100 feet in diameter.

Cabral has been studying the site, near the village of Calcoene, just north of the equator in Amapa state in far northern Brazil, since last year. She believes it was once inhabited by the ancestors of the Palikur Indians, and while the blocks have not yet been submitted to carbon dating, she says pottery shards near the site indicate they are pre-Columbian and maybe older – as much as 2,000 years old. Last month, archaeologists working on a hillside north of Lima, Peru, announced the discovery of the oldest astronomical observatory in the Western Hemisphere – giant stone carvings, apparently 4,200 years old, that align with sunrise and sunset on Dec. 21.