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Rome: Horologium


The obelisk in front of the Palazzo Montecitorio. Photo Jona Lendering. Horologium Augusti: sun dial in Rome, which had as gnomon an obelisk from Egypt.

One of the most remarkable monuments in imperial Rome is the Horologium Augusti, a giant sundial, which had as its gnomon an obelisk that was originally made for a pharaoh named Psammetichus II (Neferibra Psamtik, 595-589). The sun dial was created for the emperor Augustus, who had conquered Egypt and had seized the monolith in Heliopolis. The obelisk, 21.79 meter high, has been moved a bit and is now standing in front of the Italian parliament and is again used as a sundial (a line in the pavement indicates the date). The monument is described by Pliny the Elder:

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Augustus used the obelisk in the Field of Mars in a remarkable way - namely to cast a shadow and thus mark the length of days and nights. A paved area was laid out commensurate with the height of the monolith in such a way that the shadow at noon on the shortest day might extend to the edge of the paving. As the shadow grew shorter and longer again, it was measured by bronze rods fixed in the paving. This device deserves study; it was the result of a brainwave of Facundus Novius.
[Pliny, Natural history 36.72;
tr. John Healy]

Model of the Campus Martius. Museo nazionale della civilta romana, Roma (Italy). Photo Jona Lendering. Model of the Horologium. Koninklijke musea voor kunst en geschiedenis, Brussel (Belgium). Photo Jona Lendering. A modern representation of the Horologium, Via di Campo Marzio. Photo Jona Lendering.
Model of the Campus Martius (Museo nazionale della civilta romana, Rome) Model of the Horologium (Koninklijke musea voor kunst en geschiedenis, Brussels) A modern representation of the Horologium, Via di Campo Marzio.
The pinacle of the horlogium. Musei Vaticani, Roma (Italy). Photo Jona Lendering.
The pinacle of the horlogium (Vatican Museums)

The laws of physics can not be suspended and because we know the place where the obelisk originally stood, it is a matter of mathematics to establish where the original pavement with the network of bronze rods must have been. In 1976, the German archaeologist Edmund Buchner predicted the exact location of the pavement. Three years later, he did indeed find what he had been looking for in the cellar of a cafeteria in the Via di Campo Marzio.

The Horologium Augusti is, of course, shown on the famous model of ancient Rome in the Museo Nazionale della Civiltą Romana (Rome EUR). On the photo above, you can see it close to the upper edge. The small square building in the upper left corner is Augustus' Altar of Peace (Ara Pacis). On the date of the emperor's birth, the shadow of the obelisk indicated the sanctuary. A line at right angle of this shadow would cross through the center of the Mausoleum of Augustus, the big round structure at the lower edge.

A satellite photo can be found here.

© Jona Lendering for
Livius.Org, 2004
Revision: 8 Nov. 2009
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