Photos: Watch the ‘Shadow Dagger’ Solar Calendar Mark the Equinox

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.

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  1. Steve Kern

    So who chopped out that huge area of rock to form the flat surfaces on which the spots and circles were made and how did they craft it?

    1. Blake de Pastino

      Good question, Steve. All of the features are naturally occurring. Purcell told me: “The various sunlight and shadow patterns that play out across the cliff face are natural; we found no evidence that the cliff projections that create the different shadows have been modified in any way.”

  2. Kenneth Kaufman

    The roof leading to and over the disks appears to have been altered to allow the sun to hit the back wall. The roof is almost perpendicular to the wall and the almost right angle to the left side (facing) the disks also appears to have been man made. I am curious as to how it was determined that those angles were not man made. It seems odd that the sun will only enter to illuminate the disks on the one day of each equinox and that the effect and the features are completely “naturally occurring” ! What are the odds of finding such an occurrence in nature?

  3. Lee Pingel

    Most people don’t realize that determining the spring equinox is critical for planting crops. Calendars only work if there is a leap year and someone consistently marks off the days. Solar calendars are much more practical and much easier to design.

  4. Lee Pingel

    Correct me if I’m wrong, but a solar calendar should be easy to make. Place a stick in a smooth surface, like a rock. Local noon is the highest the sun is during a day. Mark the local noon of each day. The extremes of marks are the solstices. Using a string it is possible to mark the exact middle, giving equinoxes. Due to precession the actual spring equinox fluctuates between 19 to 21 March. When modern Gregorian calendar used. Precession causes the exact position of local noon on equinox and solstices to vary slightly. Exact time changes, it does not coincide exactly with local noon. But it should be good enough for planting.

  5. Kenneth Kaufman

    Lee Pingel – Respectfully, I take that the stick is a gnomon casting a shadow that you are marking at “high noon”: How do you know when the sun is at it’s highest point in the sky? Now the sun is going up, now the sun is going down? In the middle is noon? Perhaps the shadow that is cast helps to tell when it is high noon? I think that high noon would be difficult to accurately sight, by eye. Then, how do you find a naturally occurring sun dagger that just happens to produce a sun dagger on the equinox that you can depict a spiral on that spot and the dagger will illuminate the spiral on the equinoxes? The sun moves fastest around the equinoxes and slowest around the solstices. This naturally occurring effect is only produced exactly on the equinoxes. How is it possible that the ancients recognized not just when the equinox was exactly and also how to use a naturally occurring sun dagger to mark those two days of the year and only those two days?