OZARK, Mo. -- On the shortest days of the year, a Native American etching emerges from the gloom deep inside Smallin Civil War Cave, briefly illuminated by the rays of the sun.

The circular symbol with short rays emanating from its edge might be a representation of the sun and a reminder for Osage Indian hunters that now, with winter's chill about to arrive in full force, is the time to leave their Ozarks lowland hunting grounds, The Springfield News-Leader reported.

Could this be a primitive, yet highly accurate, astronomical calendar foretelling the arrival of winter? Possibly, according to Eric Fuller, staff archaeologist at Smallin Cave in the rolling hills on Ozark's east side.

"This (solstice illumination) does not happen any other time of the year," said Fuller, during a recent excursion into the popular tourist cave. "And the sun symbol is prominently in the center of the biggest formation in the cave. They took a lot of time to put the sun formation right there."

Tuesday was 2015's winter solstice, the shortest day of the year and the first day of winter.

Several days ahead of the solstice, and a few days after, the sun rises low enough on the horizon to send shafts of light into the cave. Bright sunbeams reflects off the rippling stream flowing through the bottom of the cave, shining on stalactite formations on the cave ceiling and illuminating the sun petroglyph carved into the side of a massive flowstone more than 300 feet back in the cave.

There are hints in the historic record about what that etching could be.

"There's a letter from a government agent in 1820 describing the Osage's year," Fuller said. "He wrote that after harvesting their corn in the fall, the Osage moved to Ozark hunting areas until just before Christmas, when they would leave. The winter solstice -- the shortest day of the year -- happens now, and when the sun hits this petroglyph it might be a possible indicator to them to head on back up."

Fuller said he has spoken with Osage tribal leaders in Oklahoma to learn more about the mysterious petroglyph. He said it's unclear how long it has been there, but some of the etched sun rays have been obliterated by layers of calcite deposited by water flowing down the rock, possibly for centuries.

An archaeological dig in 2014 found bits of evidence of human habitation at the cave dating back at least 7,000 years.

The petroglyph shares space on the flowstone walls with other markers of human presence in the cave: Names and dates carved into the undulating rock by pioneers, Civil War soldiers and from later visitors to the cave. The earliest visible date: 1800. The earliest signed graffiti: "J. Hannah 1849."

They probably had no idea the massive cave, more than three-fourths of a mile long and with the largest opening of any cave in Missouri, was revered as a sacred place by the Osage.

He noted Osage members typically began their day with a prayer to the east, toward the rising sun. He pointed out an Indian marker tree still growing next to the gift shop, which was bent at an early age to mark the source of water emanating from the two caves on the property.

It also points due east, directly toward the place on the horizon where the sun emerges at dawn.

"Every morning they'd get up and pray to the sun for guidance," Fuller said. "To have a place where the sky is coming into the earth, that was pretty amazing to them. They often looked for places where they could sense the presence of God, and for them a cave was a mystery place. Definitely caves were sacred spots for the Osage."

Information from: Springfield News-Leader, http://www.news-leader.com