Petra, a giant metropolis of tombs, monuments, and other elaborate religious structures carved into stone cliffs, was the capital of the Nabatean kingdom, a little-understood Middle Eastern culture that ruled much of modern-day Jordan from the third century B.C. until the first century A.D.
These wealthy spice traders worshiped the sun, among other deities, and may have given importance to the equinoxes, solstices, and other astronomical events that are determined by how the sun moves across the sky. (Also see: "'Lost City' of Petra Still Has Secrets to Reveal.")
Solstices, for instance, are the results of Earth's north-south axis being tilted 23.5 degrees relative to the plane of our solar system. This tilt causes different amounts of sunlight to reach different regions of the planet during Earth's year-long orbit around the sun.
Now, a statistical analysis published in the Nexus Network Journal reveals that these heavenly phenomena likely influenced how the Nabateans created structures at Petra, a Greek word that means "rock."
"The facades of Petra are not only beautiful in themselves, but they also show something additional," said study leader Juan Antonio Belmonte, an archaeoastronomer at the Institute of Astrophysics of the Canary Islands (IAC), in a phone interview.
Sanctified by the Sky
For the study, Belmonte and his colleagues measured the spatial orientations of large monuments, temples, and sacred tombs and compared the measurements with how the structures aligned with the position of the sun on the horizon.
Because that position changes very slowly through time, the amount of change between the first century B.C. and today is small—so what Belmonte and his team saw was very close to what the Nabateans would have observed.
The results of their study showed that during certain times of the year, such as the winter solstice, the sun would highlight or align with some of the city's most important buildings. (See National Geographic's Petra pictures.)
Though the team relied on statistics to confirm the sun's orientation with the monuments, the Nabateans didn't necessarily need to know mathematics—they could have built structures to align with the sun simply by observing sunrises and sunsets during significant times of year, he noted.
One of Belmonte's most fascinating discoveries is tied to the winter solstice, which the Nabateans may have believed was related to the birth of their main god, Dushara, he said. (Also see: "Before & After: Wine-Cult Cave Art Restored in Petra?")
During the winter solstice in Petra, the setting sun creates effects of light and shadow around a sacred podium inside the monument known as Ad Deir, or the Monastery, where the Nabateans may have held religious festivities.
"It's the same thing seen in Christian churches [when sunlight illuminates] special altars," Belmonte said.
E. C. Krupp, director of the Griffith Observatory in Los Angeles, called this Ad Deir effect "fetching." The fact that it manifests only a week before and a week after the winter solstice suggests that "a symbolic alignment with the winter solstice sunset is plausible," he said.
"This demonstrates we are not looking at an ancient observatory, but at architecture that is in part activated and sanctified by the sky," Krupp said in an email.
The study draws on the "well-laid-down tradition of using statistics to decide astronomical orientation," said astronomer and anthropologist Anthony F. Aveni.
"The analysis and measurements are sound," said Aveni, a professor at Colgate University in Hamilton, New York.
Though the results are intriguing, it's tough to prove that the Nabateans deliberately built their city with the sky in mind, noted Krupp and Aveni, who were not part of the study.
For one, knowledge of Nabataean traditions and ideology is limited, mostly because the culture left very few written documents.
What's more, "there aren't other related examples that can be used for comparison," said Krupp.
Belmonte begs to differ, noting that the city of Hegra, or Mada'in Saleh, in present-day Saudi Arabia "would be a marvelous laboratory to test our discoveries."
According to Aveni, this kind of research could be helpful in deciphering other cultures that lack written history, such as the Inca or Aztec cultures.
In the absence of a written record, he said, scientists can use "architectural and celestial clues to learn something about the ideologies of ancient civilizations."
Study leader Belmonte believes that his team's research has shed new light on the little-understood ancient city, 85 percent of which remains unexcavated. He calls Petra one of the "most special places in the world," adding that "these [structures] are such huge marvels of human ability created with a sense of beauty, which is related to the sky."