It took nearly 85 years for archaeologists to figure it out, but an inscribed cliff face in Arizona‘s Wupatki National Monument turns out to be a kind of 800-year-old timepiece, whose only moving parts are the orbit of the Earth and the wheeling of the sun through the sky.
First recorded in 1931 by anthropologist Harold Colton, the petroglyphs found along a landform known as Horseshoe Mesa remained poorly understood for much of the 20th century.
“The original 1931 records that Harold Colton created consisted of a 3-by-5 index card with one or two sentences ‘describing’ the site,” said David Purcell, a supervisory archaeologist at the Museum of Northern Arizona who’s leading a new re-investigation of the site.
After the 1930s, Horseshoe Mesa went largely unstudied until it was surveyed in the early 1980s, and researched again in the ’90s, but shortages of time and money conspired to keep the meaning of its petroglyphs a secret, at least for a time.
Then in 2015, after extensive study, collaboration with experts in prehistoric rock art, and uncounted hours documenting the petroglyphs with time-lapse photography and video, researchers say they’ve confirmed that the sandstone face known as Panel 50 is “definitely an observatory for the winter solstice and equinoxes.”
“Prehistoric observatories can measure important astronomical events through sighting or imaging,” Purcell explained.
“Sighting utilizes distant landmarks on the horizon to mark solar, lunar, planetary, or stellar risings and settings.”
Stonehenge is probably the most famous example of a sighting observatory, he noted. [See a striking alignment found in America’s largest prehistoric city: “Ceremonial ‘Axis’ Road Discovered in Heart of Ancient City of Cahokia“]
“Imaging is the observation of sun or moon rays or shadows projected on a surface,” Purcell added.
“Panel 50 is an imaging calendar.”
Because of the natural happenstances of its orientation and its features, Panel 50 creates a unique interplay of light and shadow around the beginning of spring, fall, and winter.
“People living at Horseshoe Mesa would have observed these patterns and realized that they could be used to identify important solar milestones like the equinoxes, with the addition of the petroglyph elements,” Purcell said.
A natural outcropping of rock above the panel forms what researchers have dubbed a “shadow dagger” that bisects a spiral carved onto the cliff wall, while another shadow interacts with a set of eight circles pecked into the panel’s left side.
“Because the two elements appear to function together to measure time, we believe that they were added to the cliff face at the same time,” Purcell said. [Learn about the annual observations made at Panel 50: “Thousands of Ancient Petroglyphs, ‘Dramatic’ Solar Calendar Reported in N. Arizona“]
The sunwatchers who made these modifications were members of the Kayenta tradition, a group of Ancestral Puebloans who occupied northern Arizona from around 500 to 1300 CE, and lived at Horseshoe Mesa from the mid-12th to the beginning of the 14th century.
And while the “shadow dagger” site may have distant connections to similar observatories like those once found at Chaco Canyon, Purcell pointed out that the timekeeping done at Panel 50 is by no means unique to the Ancestral Puebloans. [Read about the role of meteorites at ancient sites: “Prehistoric Meteorite ‘Shrines’ in Arizona May Be Linked, Says Archaeo-Astronomer“]
“Several solar calendars have been documented in the Verde Valley, which suggests that the knowledge of solar observation was greater than just the Chaco region, extending into the Sinagua and Hohokam areas,” he said.
The photos below show the observations made at Panel 50 on the spring equinox of 2015, as well as some of the artifacts found at Horseshoe Mesa.
Click any image to enlarge.