bbc.com / All day the sun had been hiding behind clouds, but Kenneth Zoll wasn’t worried.
Standing in front of panels of rock art carved nearly a millennium ago, the researcher pointed to petroglyphs of snakes, coyote and deer, and singled out several concentric circles. Then he told his 100-strong audience to note the two rocks wedged into a crack above his head.
Zoll swept his arm across the entire scene. “This,” he explained, “was a way to track time.”
This was a way to track time
As if by command, a few minutes later the clouds began to part. And at 13:40, like a perfectly calibrated Swiss watch, a beam of light passed over the rocks and projected two shadows across the panel. For the next six minutes, their edge held still, just touching three circles.
Spring had arrived in Arizona’s Verde Valley.
On this day, March 20, at archaeological sites around the globe, from Mexico’s Chichen Itza to Malta’s Mnajdra temple, something similar was happening. On the vernal equinox, ancient ruins were aligning with the sun, and – whether anyone was watching or not – they were silently marking the changing season.
View image of Petroglyphs in Arizona’s Coconino National Forest were carved to track the passage of time (Credit: Credit: Larry Bleiberg)
You may also be interested in: • The mystery behind Greece’s temples • The clock that changed the meaning of time • The tiny island of giant money
When the sun emerged here at the Coconino National Forest near Sedona, Arizona, the crowd began to buzz with excitement. Susie Reed, a local photographer, said she felt it was important to see the rock mark this day, when the sun passes over the equator. “We keep the energy alive by coming out here.”
But until the last decade, it had been centuries since anyone noticed.
We keep the energy alive by coming out here
In 2005, Zoll, then 57 and a volunteer at the forest’s V Bar V historical ranch site, detected a pattern to the shadows cast on the park’s huge rock art panels, which are covered with more than 1,000 petroglyphs.
Could this, he wondered, be an ancient calendar?
He shared his observation with a forest service archaeologist, who wasn’t particularly impressed. Archaeo- or cultural astronomy, the study of how ancient peoples tracked the seasons and studied the cosmos, has fought for respectability. It’s hard to prove that alignment with the sun, moon or stars isn’t mere coincidence. And in the past, some advocates haven’t helped their case, suggesting that prehistoric sites could have been fashioned by space aliens.
View image of Kenneth Zoll kept careful records of how the sun and the shadows fell over the petroglyphs of the V Bar V historical ranch site (Credit: Credit: Larry Bleiberg)
But in the last decades, scholars have shown that societies once considered primitive actively monitored celestial events, and Unesco has begun to recognise the astronomical heritage of sites such as Chaco Culture National Historical Park in New Mexico and England’s Stonehenge.
And while the rock art at V Bar V isn’t nearly as grand as more famous markers, Zoll thought it too held a secret. The forest service scientist told him to observe the site for a year and then get back to him.
Undaunted, the former computer systems manager began to watch and keep careful records. “I went from high tech of the 20th Century to high tech of the 11th Century,” he said.
And what he found astonished him.
View image of Zoll believes the Sunagua people created the calendar to mark religious ceremonies and track the growing seasons (Credit: Credit: U.S. Forest Service Coconino National Forest)
Every month, as the sun interacted with the art, the rock, it seemed, was speaking to him. On the summer solstice, which falls on 21 June this year, shadows interplayed with half a dozen images on the stone. Six months later, on the shortest day of the year, the sun shone directly through a notch between two rocks.
It was agriculture that likely inspired the so-called imaging calendar, Zoll said. The Sinagua people, who researchers believe lived and farmed here between the 7th and 15th Centuries, grew corn, cotton, squash and beans. Their descendants, the Hopi people, now live about 150 miles away.
It’s a time machine for the gods
When Zoll spoke to the Hopi, he learned that the panel appeared to mark religious celebrations and important dates for farmers. On 21 April, a day associated with the first planting, a shadow’s bottom edge touches a carving resembling a corn stalk. One of the most compelling findings comes on 8 July, the end of a 16-day Hopi period of prayer and meditation. On that day, the sun perfectly outlines a figure that appears to be dancing.
“It’s a time machine for the gods to tell you it’s time for a ceremony,” said Scott Newth, an officer with the Arizona Archaeological Society, who tracks rock art across the region.
Hopi elder Floyd Lomakuyvaya, 65, said that some of the petroglyphs are the familiar symbols of tribal clans. “I feel proud because our ancestors left these marks. Every month we have different ceremonies, and different things happen. That’s our calendar. It guides us.”
And on this year on the vernal equinox, Hopis were in full-force at the rock art site. They had brought a youth group to learn about their heritage and to help with demonstrations, including an agave pit roast. The sizeable head of the desert plant was a staple for the tribe, providing sustenance throughout the year.
That’s our calendar – it guides us
Zoll has documented a dozen sites in the Verde Valley area of central Arizona that are likely to be calendars, and more than 30 others have been found in the region around the city of Phoenix, he said. Most have rock art with concentric circles, which appear to align with shadows at specific times of year. “We always wondered why we’d see the exact same images.”
He says one working theory is that outsiders travelled through the region, teaching locals how to construct the calendars. This is based on the discovery of a northern Arizona burial of a man who appeared to be a visitor because his body was larger than others found in the area. He was buried with a medallion etched with concentric circles.
View image of More than 30 other petroglyph examples believed to be ancient rock calendars have been found in central Arizona (Credit: Credit: U.S. Forest Service Coconino National Forest)
Researchers have also discovered several observation sites in the area that appear to have been used by sun watchers, tribal members who were tasked with observing the daily rise and setting of the sun. It’s a position of honour that still exists in Hopi communities, where members of the Water Clan attend to that duty. Indeed, turtles, which are their clan symbol, appear to be carved above the V Bar V rock. As the minutes ticked by that afternoon, the shadows grew more distinct on the rock face, and then they slowly started to fade away.
Zoll didn’t seem surprised that the sky had cleared just in time for the display. Early in his research, something similar occurred during a solstice. It had been overcast all day and then suddenly the sun emerged, just in time for him to record its shadow. A few minutes later, the clouds returned.
When he recounted the story later, a Hopi man offered an explanation: “The creator wanted you to see that.”
View image of According to forest ranger Terrilyn Green, the migration pattern of the common black hawk is another way to track time (Credit: Credit: S.B. Nace/Getty Images)
Forest ranger Terrilyn Green, who was supervising the equinox event, said she’s pleased how the rock art’s fame is growing, but notes there are many ways to track time.
Every year, the common black hawk returns from its winter migration to Arizona’s Verde Valley in late March. “It’s amazing. It’s like a wonderful harbinger of spring,” Green said.
And that morning, like clockwork, she heard the raptor’s call for the first time this year.
Join more than three million BBC Travel fans by liking us on Facebook , or follow us on Twitter and Instagram .
If you liked this story, sign up for the weekly bbc.com features newsletter called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital and Travel, delivered to your inbox every Friday.