engraved stones
© BERNARDI, ET AL
One of the engraved stones on the elevated site at La Silla, thought to be part of an ancient star-observing platform.
The complex astronomical measurements that underpinned many aspects of the Inca civilisation may have an ancient forerunner of 10 centuries earlier and 2000 kilometres distant, a prominent archeoastronomer suggests.

Steven Gullberg, of the University of Oklahoma, US, and chair of the International Astronomical Union Working Group for Archaeoastronomy and Astronomy in Culture, is the latest scientist to comment on the origin and purpose of some mysterious stones and engravings, known as petroglyphs, at a site known as La Silla, in Chile.

The complex designs etched in rock, together with a set of standing stones, were first studied in depth by researchers - including this writer - in 2012.

It was suggested that the artefacts were set up to mark the positions of two very brilliant stars, Canopus and Hadar, and were the work of the El Molle, a pre-Columbian culture that occupied the region for five centuries from about 300 CE.

Curiously, La Silla is today the site of a facility built and operated by the European Southern Observatory (ESO), an important part of the global infrastructure for astronomers.

If the tentative conclusions about the petroglyphs and standing stones are correct, the site has been a critical place for star-gazers for at least 1700 years.

According to Gullberg, it conveniently fits into one significant question about Incan culture - namely, how a civilisation that lasted for only a short period of time managed to accumulate a vast body of knowledge about astronomy.

"This empire lasted for less than 100 years," he says, "and its knowledge, not only astronomical, was already well-established prior to its foundation in 1438; this means that the roots of the astronomical culture of the region have to be found in more ancient periods.

"So, it is not strange that ancient traces can be found in times as remote as those of the El Molle culture, which is separated from the Incas by a distance of 2000 kilometres and of several centuries in time."

The El Molle community, left a hunting lifestyle to adopt farming and herding. This is evident from the images carved in the La Silla rocks, which include abstract figures and large herds of llama-like animals. Today, the region is one of the most arid deserts of the world, but these findings, and other geological clues, show that at the time the area was much less barren.

In a second research paper, published in 2014, we were able to rule out some of the most obvious astronomical interpretations of the artefacts - in particular that they were constructed to observe solstices and equinoxes, the moon or the planets.

The only scenario that fitted, instead, was the pinpointing of Canopus and Hadar, two of the brightest stars of the southern hemisphere, when they appeared aligned and close to the horizon.

These particular stars may have been significant to the El Molle people because although their alignment happens every night, for two very short periods it occurs just before sunrise. One of these, dubbed the heliacal rise and set by the astronomers, coincides with the season change from warm to cold, and might thus have signalled the correct time to move flocks from high pastures to lower ground.

Ironically, we identified the importance of Canopus and Hadar, and their connection with this ancient people, because the standing stones are no longer accurate.

The altitude of the alignment on the sky, in fact, changes over time because of the precession of the equinoxes, a 25,600-year-long movement of the Earth's spin axis that varies the orientation of the planet with respect to the stars.

Calculations show that the heliacal rise and set happened approximately between 100 and 800 CE. That is in the same period in which the people of the El Molle culture inhabited this territory.

The accuracy of current measurements might not be sufficient to establish beyond any reasonable doubt the astronomical interpretation of this site, and, probably, even a complex campaign conducted with more sophisticated instrumentation wouldn't be able to settle completely the debate.

Nevertheless, Gullberg suggests that searching for other places with similar connections would be a good way of "strengthening the overall scenario suggested by the research, and overcoming some of the current limitations that are unavoidable in this field".

It is possible that the final answer will never be found, but it is certainly appealing to think that, 1500 years later, modern astronomers might have chosen the same place as their distant ancestors to question the sky.