català | castellano | english
logo news
el sol i el temps | news >> Horologium Augusti, the largest sundial in the ancient world, whose remains are still visible in Rome [Guillermo Carvajal - La Brújula Verde]

Horologium Augusti, the largest sundial in the ancient world, whose remains are still visible in Rome [Guillermo Carvajal - La Brújula Verde]

In 10 BC. Emperor Augustus commissioned the mathematician and architect Novius Facundus to design and build a sundial in Rome, the Horologium Augusti, as a commemoration of the Roman rule of Egypt and of Augustus himself born to bring peace to the world.

Thus, its installation was projected in the Field of Mars integrated with the Ara Pacis (Altar of Peace) that was being built since 13 BC. and aligned with the Via Flaminia. To use as a gnomon (the element that casts the shadow) Augustus had a 30-meter-high red granite obelisk brought from Heliopolis in Egypt, which had been erected by Pharaoh Psammetico II between 595 and 589 BC.

The obelisk cast its shadow on a quadrangular marble pavement measuring 160 by 75 meters, composed of travertine slabs, with inlaid gilt bronze lines and letters that indicated the months and the seasons. It was placed in such a way that the shadow fell over the center of the Ara Pacis on September 23, the autumn equinox and Augustus' own birthday. The clock was completed in 9 BC. and it was dedicated to the Sun.

Pliny the Elder describes it when he talks about the obelisks of Rome:

The one found on the Field of Mars was used in a remarkable way by Augustus of venerated memory to mark the shadow of the sun and with it the duration of the days and nights. A pavement was placed at a suitable distance at the height of the obelisk so that the shadow cast at noon on the shortest day of the year coincided exactly with it. Some bronze rods inserted into the pavement were used to measure the shade day by day, as it was getting shorter and longer. This device deserves to be studied carefully, and was devised by the mathematician Novius Facundus. Pliny the Elder, Natural History XXXVI.71

The area covered by the Horologium Augusti on the Champ de Mars corresponds to a circle that encompasses the current Piazza di San Lorenzo in Lucina and the Piazza del Parliament. The obelisk, which stood in the center of this circle, was topped with a bronze orb. Depending on the position of the sun, the shadow of the obelisk was projected on one side or the other of the marble quadrant, indicating the day of the month according to the length of the shadow at noon.

According to Pliny, the orb was what gave definition to the shadow, something that Novius understood by observing the shadow cast by the human head.

He placed on the pinnacle a golden ball, at the top of which the shadow would be concentrated, since otherwise the shadow cast by the tip of the obelisk would have lacked definition. It is said that he understood the principle from observing the shadow cast by the human head. Pliny the Elder, Natural History XXXVI.71

But by the time Pliny was writing, around A.D. 70, the watch had not worked correctly for several years, for which he gives different explanations. The truth is that the excavations found that the soil of the Champ de Mars had sunk, probably due to floods or earthquakes.

For about thirty years, the readings thus given do not correspond to the calendar, either because the course of the sun itself is anomalous and has been altered by some change in the behavior of the heavens or because the entire The earth has shifted slightly from its central position, a phenomenon that I have heard has been detected elsewhere as well. Either the earthquakes in the city may have caused a purely local displacement of the shaft or the floods of the Tiber may have caused the settlement of the mass, although the foundations are said to have sunk to a depth equal to the height of the burden they have to bear. Pliny the Elder, Natural History XXXVI.71

To compensate for the sinking of the ground and restore the clock to its accuracy, Domitian raised the level by more than a meter and a half, using the pavement and the original letters of the Horologium.

Some researchers believe that it was not a sundial but a solar meridian (which does not mark the time but only the noon or meridianus) designed to indicate the progress of the year as the sun moved through the zodiac, from solstice to solstice. Instead of the travertine pavement, there would only have been a longitudinal line that would delimit the greatest extension of the shadow at noon on the winter solstice. At the summer solstice, the longest day of the year, the shadow would have completely moved along the meridian line. Then, as the sun descends on the horizon, its noon shadow would begin to lengthen and move up the meridian until it can no longer grow, thus marking the winter solstice, the shortest day of the year. The obelisk remained standing until the 9th or 10th century when, probably during the 849 AD earthquake. Or perhaps because of the looting in 1084 AD, it fell to the ground and broke into five pieces, gradually being covered by sediment. It was found in 1512, but was not excavated until 1748. The remains were recovered and the obelisk rebuilt (with sections of red granite taken from the heavily damaged Antonine column) and re-erected by Pius VI in 1792 in the Plaza Montecitorio. from Rome, where we can see it today. It is 21.79 meters high, reaching 33.97 if the base and the orb are added. In 1980 Edmund Buchner, historian who was president of the German Archaeological Institute, found the remains of a small section of pavement with the Greek lines and letters of the Horologium Augusti months etched into the travertine slabs under the house blocks between the Plaza del Parliament and the Plaza de San Lorenzo in Lucina, 8 meters deep. Just as Pliny had described. Another fragment of the pavement is believed to be contained in the mosaic, still visible, in the foundations of the church of San Lorenzo in Lucina.

The reconstruction of the Horologium proposed by Buchner, and his theory that the shadow of the obelisk fell on the Ara Pacis on Augustus' birthday, have been questioned by some researchers, while others argue that both monuments were aligned and intentionally placed to propagate the same message that appears inscribed on two faces of the obelisk: that Augustus was a devoted worshiper of the Sun god, the one who leads Rome to victory and brings peace and prosperity through his earthly representative, the emperor.

In the reform of the Montecitorio square inaugurated on June 7, 1998, a new meridian was drawn on the pavement in honor of Horologium Augusti, although the shadow of the obelisk no longer marks the months or the seasons.


The Green Compass / Horologium of Augustus - Encyclopaedia Romana (James Grout) / Frischer, B. (2017). New Light on the Relationship between the Montecitorio Obelisk and Ara Pacis of Augustus. Studies in Digital Heritage, 1 (1), 18–119. / Natural History (Pliny the Elder) / Heslin, P. (2007). Augustus, Domitian and the So-Called Horologium Augusti. The Journal of Roman Studies, 97, 1–20. / Wikipedia


Related photos

Location of the Horologium Augusti in the map of Rome / photo Joris1919 in Wikimedia Commons
Model that reconstructs the Field of Mars with the Ara Pacis and the Horologium Augusti / photo Pascal Radigue in Wikimedia Commons
Nineteenth-century reconstruction of the situation at the Horologium Augusti / photo James Grout
The obelisk of the horologium represented at the base of Antonino Pio's column, today in the garden of the Vatican Museums / photo Lalupa in Wikimedia Commons
The obelisk in Piazza Montecitorio / public domain photo in Wikimedia Commons
The remains of the pavement found by Buchner / photo Lalupa in Wikimedia Commons

Last news
:: 17/12/2021 - Parets Town Hall rebuilds the Marineta sundial from a 1924 photo
:: 16/07/2021 - How did they measure time in ancient times?
:: 06/07/2021 - The Tristaina solar viewpoint opens to the public next Friday
:: 20/06/2021 - BSS - The Bulletin Archive
:: 19/05/2021 - Horologium Augusti, the largest sundial in the ancient world, whose remains are still visible in Rome [Guillermo Carvajal - La Brújula Verde]
(59 news - look repository)

Creative Commons License

The webside El sol i el temps and the content is under licence of Attribution-Noncommercial-No Derivative Works 3.0 Spain Creative Commons

logo El sol i el temps
:: (c) 2014-2023 El sol i el temps
:: Last update: July 10, 2023 | Design by: BitWorks
:: Web map | Contact | Legal Note