A University of Colorado Boulder faculty member says a petroglyph in New Mexico’s Chaco Canyon might represent a total eclipse that occurred there 1,000 years ago.
The petroglyph depicts a circle that resembles the sun’s outer atmosphere, or corona, with tangled protrusions looping off its edge, according to a news release from the university.
The petroglyph was discovered in 1992 during a Chaco Canyon field school for CU Boulder and Fort Lewis College students led by CU Professor Emeritus J. McKim “Kim” Malville and then-Fort Lewis professor James Judge, according to CU.
Malville, of CU Boulder’s astrophysical and planetary sciences department, said the object may illustrate the total solar eclipse on July 11, 1097.
“To me it looks like a circular feature with curved tangles and structures,” Malville said in the news release. “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’s circle may illustrate a coronal mass ejection, an eruption that can blow billions of tons of plasma from the sun at several million mph. But if the sun was in a “quiet phase” of its 11-year cycle, a coronal ejection would be unlikely, Malville said.
“It turns out the sun was in a period of very high solar activity (at the time of the 1097 eclipse), consistent with an active corona and CMEs,” Malville said.
Malville and professor José Vaquero, of the University of Extremadura in Cáceres, Spain, published a paper on the petroglyph in the Journal of Mediterranean Archaeology and Archaeometry in 2014.
Malville and Vaquero used data from ancient tree rings, including their amount of the isotope carbon-14 to correlate the time of sunspots – the less carbon-14, the more sunspots. More sunspots indicate higher solar activity, Malville said.
Malville and Vaquero also used records of naked-eye observations of sunspots, and looked at historical data compiled by northern Europeans on the annual number of nights when the northern lights were visible, an indication of intense solar activity.
Piedra del Sol, the rock that bears the petroglyph, also has a large spiral petroglyph on its east side that marks sunrise 15 to 17 days before the June solstice, 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, he said.
The east side of Piedra del Sol contains a bowl-shape depression where Chacoans likely left offerings such as cornmeal. The southwest side of the rock faces a butte on the horizon that marks the December solstice event, and the rock also has carved steps, indicating that it likely had some kind of a ceremonial importance, he said.
“This possible eclipse petroglyph on Piedra del Sol is the only one we know of in Chaco Canyon,” Malville said. “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.”