The Shadow of the Solarium Augusti

23 September - The shadow cast by the Solarium Augusti, the giant sundial that once stood in Rome’s Campus Martius, passed over the centre of the monumental Altar of Augustan Peace, marking the deified emperor Augustus’ birthday and autumn equinox.

“The one in the Campus was put to use in a remarkable way by Augustus of Revered Memory so as to mark the sun's shadow and thereby the lengths of days and nights. A pavement was laid down for a distance appropriate to the height of the obelisk so that the shadow cast at noon on the shortest day of the year might exactly coincide with it. Bronze rods let into the pavement were meant to measure the shadow day by day as it gradually became shorter and then lengthened again. This device deserves to be carefully studied, and was contrived by the mathematician Novius Facundus. He placed on the pinnacle a gilt ball, at the top of which the shadow would be concentrated, for otherwise the shadow cast by the tip of the obelisk would have lacked definition. He is said to have understood the principle from observing the shadow cast by the human head.” (Pliny the Elder, “Natural History”)

An early 19th century attempt to imagine the Solarium Augusti on Campus Martius
with the altar to the right by the side of Via Flaminia, the Tiber to the left and Augustus’ mausoleum in the background.

The pinnacle Pliny the Elder refers to is actually the tip of a 70’ red granite obelisk from Heliopolis in Egypt, brought to Rome by Augustus after the conquest. The trophy became an additional, delicate detail, not only by its use as, probably, a sundial, but by its integration to the Ara Pacis Augustae, the Altar of Augustan Peace. Consecrated after a building time of four years in 9 BCE, the altar stood near the old vital traffic line of the Via Flaminia on the outskirts of Rome and was a 35’ wide and 18’ high masterpiece of Roman sculpture from the days of the late Republic, created by Greek artists. The complex itself was a marble altar framed by stone walls on a marble base, all heavily ornamented with reliefs. Celebrating the end of the wars, the Roman Senate pronounced its final submission under the sole rule of the emperor, as well as the recovered adherence to Roman tradition and piety by having scenes of classical Roman mythology depicted and connected with Augustus’ autocracy. A pace setter for the emperors to come.

The Ara Pacis Augustae

The Solarium Augusti with the obelisk as gnomon, the part of a sundial that casts the shadow, had provably at least 20’ meridian lines to use it over the year as a calendar and a noon mark. If the Solarium – or horologium, meaning, more or less, a clock – really was a sundial is disputed. The rumoured dial-plate that allegedly covered 600’ around the Solarium had, at least, never been discovered. But besides the astronomical use, the ensemble had a subtle propagandistic purpose. On Augustus' birthday that concurred with the autumn equinox, the gnomon’s shadow would wander across the Campus Martius until, at noon, it reached the altar in the centre of the Ara Pacis Augustae, as if Sol, the live-giving sun, would lay his hand on it himself, a celestial sign, that Augustus was born to bring peace to Rome. Which he indeed did to a certain extent.

The Mausoleum of Augustus today

Added to the ensemble was Augustus’ mausoleum, continued to amaze and impress travellers on the Via Flaminia and visitors of the other wonders on the Campus and the complex survived both sacks of Rome by Alaric and Genseric during the 5th century until at least the 700s, when the Western Empire had long been fallen and Rome had become the Papal metropolis by the grace of the Carolingian Franks. Then, the installations were lost when the Campus Martius became Rome’s most populated quarter during the Middle Ages. But the obelisk and parts of the altar’s frieze resurfaced during the 16th century and the obelisk was re-erected on Piazza Montecitorio in 1789, 500 yards away from its original site, while reconstruction of the altar began in 1903. Today, the obelisk faces the Italian Chamber of Deputies and a meridian has been embedded in the pavement, pointing at the Chamber’s main entrance. The shadow play, unfortunately, does not perform any longer and the effect lost in Rome’s past.

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