A petroglyph on the south face of Piedra del Sol, a free-standing rock in New Mexico’s Chaco Canyon, may depict the solar corona observed during the total solar eclipse of July 11, 1097 CE, says Professor J. McKim ‘Kim’ Malville of the University of Colorado, Boulder.

The petroglyph — carved in a rock by early Pueblo people — is an unusual circle with curved rays, which resemble the curved coronal structures of the eclipsed Sun during a coronal mass ejection (CME) or, even, exaggerations of curved polar streamers of the corona.

It was discovered in 1992 by Professor Malville and Fort Lewis College Professor James Judge.

“To me it looks like a circular feature with curved tangles and structures,” Professor Malville said.

“If one looks at a drawing by a German astronomer of the 1860 total solar eclipse during high solar activity, rays and loops similar to those depicted in the Chaco petroglyph are visible.”

“One tangled loop jutting from the petroglyph circle may illustrate a CME, an eruption that can blow billions of tons of plasma from the Sun at several million miles per hour during active solar maximum periods.”

“But if the Sun was in a ‘quiet phase’ of its roughly 11-year cycle, one would expect few if any CMEs, and the likelihood of one occurring during a solar eclipse would be negligible.”

“This was a testable hypothesis. It turns out the Sun was in a period of very high solar activity at that time, consistent with an active corona and CMEs,” Professor Malville said.

In the recent study, Professor Malville and his colleague, Professor José Vaquero of the University of Extremadura, Spain, used several sources to assess the activity of the Sun around the time of the 1097 eclipse.

That included data from ancient tree rings, formed annually and which have been cross-dated to create time series going back thousands of years and which also contain traces of the isotope carbon-14.

“Created by cosmic rays hitting Earth’s atmosphere, carbon-14 amounts in the tree rings can be correlated with sunspots — the less carbon-14, the more sunspots, which indicates higher solar activity,” Professor Malville explained.

The researchers also used records of naked-eye observations of sunspots, which go back several thousand years in China.

In addition, they looked at historical data compiled by northern Europeans on the annual number of so-called ‘auroral nights,’ when the northern lights were visible, an indication of intense solar activity.

“Piedra del Sol also has a large spiral petroglyph on its east side that marks sunrise 15 to 17 days before the June solstice,” Professor Malville said.

“A triangular shadow cast by a large rock on the horizon crosses the center of the spiral at that time. Such a phenomenon may have been used to start a countdown to the summer solstice and related festivities.”

“In addition to the spiral petroglyph, the east side of Piedra del Sol contains a bowl-shaped depression where Chacoans likely left offerings like cornmeal,” he said.

“The southwest side of the rock faces a small butte on the horizon that marks the December solstice event, and the rock also has carved steps, indicating it likely had some kind of a ceremonial importance.”

“This possible eclipse petroglyph on Piedra del Sol is the only one we know of in Chaco Canyon,” Professor Malville noted.

“I think it is quite possible that the Chacoan people may have congregated around Piedra del Sol at certain times of the year and were watching the Sun move away from the summer solstice when the eclipse occurred.”

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J.M. Vaquero & J. McKim Malville. 2014. On the Solar Corona Petroglyph in Chaco Canyon. Journal of Mediterranean Archaeology and Archaeometry 14 (3): 189-196