The site 19 miles north of Cedar City off state Route 130 is believed to have been used by members of the ancient Fremont culture to mark the longest day of the year - Tuesday this year - when the sun is farthest north on the horizon.

Rock faces at the site contain a plethora of petroglyphs that some researchers say explain how the site was used as a calendar system by the culture that inhabited the area 1,000 years ago.

"It allowed the people to know when to plant their crops and have babies," said Nal Morris, an instructor of archaeoastronomy at Utah Valley State College in Orem, who spoke to the crowd.

The gathering at the gap on Saturday was sponsored by the Bureau of Land Management and the Parowan Historical Society.

Archaeologists recently completed a 10-year survey of the area and found a wealth of information for further study, according to archaeologist Garth Norman, who led the efforts.

Norman said 50 areas of significance were found, including habitat sites, hunting places and evidence the people who once lived in the area were familiar with the Mesoamerica calendar used by ancient cultures in the southwest and Mexico.

Todd Prince, the manager of the Iron Mission State Park in Cedar City, told the crowd that ancient cultures throughout the southwest used architecture or natural landmarks to note astronomical events through observation of light and shadow.

Evidence suggests settlements at Chaco Canyon in New Mexico, Mesa Verde in southwestern Colorado, Hoven Weep in southeastern Utah and other sites also contain structures where astronomical events were noted, he said.

"They knew the night sky a lot better than we do in our lighted cities and houses," said Prince. "Their survival depended on it."

After the lecture, the crowd walked into a field where it's believed ancient people constructed rock piles from which to observe the phenomenon of the sun setting in the center of the gap.

Linda Michael, a 15-year resident of Parowan, said her family was in town for Father's Day and a trip to the gap on Saturday night seemed like a constructive activity for everyone.

"I can't live in Parowan and not know more about this," she said as the sun sank deeper into the gap.

John Fullmer, Cedar City, who attended Saturday's event, said the name of his his ancestor, David Fullmer, is on a plaque at the gap commemorating the first white settlers to discover the petroglyphs in 1850.

"This has been really interesting," said Fullmer. "I've always been interested in the archaic people and how they figured out everything they did."

Morris said in addition to the sun setting in the center of the gap on Tuesday, the moon has completed a 19-year cycle this year and will rise at 9:44 p.m. Tuesday from a notch in line with what he claims is a petroglyph panel explaining the lunar event. In addition, the planets Mercury, Venus and Saturn will also line up Tuesday night in the gap.

"It won't happen again for 19 years," said Morris. "It's a grand slam for the gap."

mhavnes@sltrib.com