History In The Headlines

Mystery of Ancient Roman Sundial Deciphered Using Digital Modeling

By Evan Andrews
In the early years of the Roman Empire, the Roman Senate ordered the construction of a marble altar to commemorate an era of tranquility ushered in by the rule of Emperor Augustus. Scholars have long thought that this Ara Pacis, or “Altar of Peace,” was designed to line up with a 71-foot-high Egyptian obelisk as part of Ancient Rome’s largest sundial, but new evidence suggests that may not have been the case. By using 3-D digital modeling and celestial data from NASA to virtually travel back in time, researchers have gained a new perspective on just how—and when—the two monuments would have interacted with the sun.
Virtual simulation image of the sun atop the obelisk with the Altar of Peace in the foreground. (Credit: Indiana University Bloomington)

Virtual simulation image of the sun atop the obelisk with the Altar of Peace in the foreground. (Credit: Indiana University Bloomington)

In a December 19 announcement at the Vatican’s Pontifical Archaeological Academy, a team of virtual archaeologists revealed that digital mapping has corrected a decades-old misconception about the famous relationship between the Ara Pacis, consecrated in 9 B.C., and the Obelisk of Montecitorio, a 6th century B.C. Egyptian relic that was brought from Heliopolis to Rome by Augustus, the empire’s founder. Scholars have long believed that the Egyptian obelisk, which acted as the pointer for a giant meridian line, was positioned to cast a symbolic shadow on the western façade of the Ara Pacis on September 23—Augustus’ birthday. But in creating a 3-D digital representation of the monuments as they would have appeared from 9 B.C. to 40 A.D., researchers have discovered that the September 23 date is incorrect.

The revelations by a team led by Bernard Frischer, a professor in the Department of Informatics at Indiana University, and John Fillwalk, Director of the Institute for Digital Intermedia Arts at Ball State University, fly in the face of a nearly 50-year-old theory by the German archaeologist and scholar Edmund Buchner, who first popularized the September 23 date. Using data from NASA’s Horizons System, an online computation tool that gives the precise location of celestial bodies from any time in history—including the age of Augustus—Frischer and Fillwalk’s group constructed a 3-D model of the Campus of Mars, the 490-acre section of Ancient Rome that housed many important public buildings including the Pantheon. While their simulations showed that the obelisk’s shadow does cross paths with the façade of the Ara Pacis on several other dates of the year, it only clips the edge of the altar on September 23.

The researchers’ models also revealed that the true significance of the interaction between the monuments was not the shadow on the Ara Pacis, but the location of the sun itself. Simulations showed that the Ara Pacis and the Obelisk of Montecitorio were aligned so that, when viewed from the nearby Via Flaminia, the obelisk would appear from behind the Ara Pacis with the sun positioned at its tip. This celestial phenomenon would have occurred not on Augustus’ birthday but on October 9, the date of the festival of the Temple of Palatine Apollo. “What’s important is not the shadow of the obelisk, but the sun’s disk seen over the center of the top of the obelisk from a position on the Via Flaminia in front of the Ara Pacis,” Frischer said in a press release. The October 9 date would have held particular significance for the Roman Empire under Augustus. Apollo, the Roman god of the sun, was also Augustus’ patron deity. The Emperor even placed the elaborate Temple of Palatine Apollo on a spot adjacent to his own home before dedicating it on October 9, 28 B.C. “No other date would have been as appropriate as this,” according to Frischer.

The revelations are just the latest developments in the long and complex histories of the Obelisk of Montecitorio and the Ara Pacis. Originally built in the 6th century B.C. during the 26th Dynasty reign of the Egyptian Pharaoh Psametik II, the obelisk was shipped to Rome at Augustus’ behest in 10 B.C. and placed in the Campus of Mars, most likely as a monument to his victory over Mark Antony and Cleopatra at the Battle of Actium two decades earlier. The structure later collapsed and was lost for hundreds of years before being rediscovered in the 15th century. Finally, in 1792, Pope Pius VI oversaw the obelisk’s restoration and placement at its current location in the Piazza Montecitorio.

The Ara Pacis was commissioned in 13 B.C. and decorated with an exquisite series of friezes to commemorate the Pax Romana, or “Roman Peace,” brought about by Augustus’ many military victories. After becoming entombed under centuries of silt, the altar was eventually excavated, reassembled and moved to its current location in the 1930s by Benito Mussolini, who erected a specially constructed pavilion to house the altar, reinterpreting it as a symbol of Italian Fascism. In the late 1990s and early 2000s, architect Richard Meier designed a new pavilion for the monument, which reopened in 2006.

Because both monuments have been moved from their original locations, Frischer and Fillwalk’s team used a combination of historical data, on the ground surveys and GPS coordinates to build their immersive archaeological model. Frischer previously used similar techniques as director of the Digital Hadrian’s Villa Project, which created a 3-D simulation of a palatial villa in Tivoli, Italy owned by the 2nd century Roman Emperor Hadrian.

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Categories: Ancient Rome