The map above is a closer look at a map of ancient Rome by Allyn and Bacon. It’s interesting because it shows the position of the Solarium, or the Horologium Augusti, a giant sundial set up by Augustus. It’s the subject of a paper in the Journal of Roman Studies by Peter Heslin: Augustus, Domitian and the So-called Horologium Augusti. This is a brilliant paper twice over. First he shows that the Horologium could not have existed as it is shown on this map. This is controversial because extremely eminent archaeologists in the German Archaeological Institute have claimed they’ve found the Horologium. This paper by Peter Heslin emphatically shows they’re wrong. Then he goes on to give an explanation of what they’ve found which makes sense archaeologically, historically and astronomically.
What is there on the Campus Martius?
The Campus Martius was an area of ground on the north side of Rome, prone to swampiness. It lay outside the the early boundaries of Rome, which meant it could be used for a variety of things not allowed in the City. It’s here the Mausoleum of Augustus is found. There are temples and altars. It was also the closest place to the walls where the army could legally muster. This made it an important area, so it’s not surprising that Augustus gave it his attention. One of the things he set up there Heslin notes is an obelisk. We have a record of the erection of the obelisk and its use from Pliny’s Natural History:
Augustus used the obelisk in the Campus Martius in a remarkable way, namely to cast a shadow and thus mark the length of days and nights. A paved area was laid out to commensurate with the height of the monolith in such a way that the shadow at noon on the shortest day might extend to the end of the paving. As the shadow gradually 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 brain wave Facundus Novius. Novius placed a gilded ball on the apex of the monolith otherwise the shadow cast would have been very indistinct. He got this idea, so it is said, from seeing the shadow cast by a man’s head. These measurements, however, have not agreed with the calendar from some 30 years. Either the sun itself is out of phase or has been altered by some change in the behavior of the heavens, or the whole earth has moved slightly off center.Translation from UOregon’s Solarium site.
It would seem that archaeologists and historians can agree that there was at least an obelisk and a line with calibrated measurements for the length of days. Beyond that people start to argue.
The opinion which holds sway at the moment is based on work by Edmund Buchner of the German Archaeological Institute. Buchner has reconstructed the Horologium. He argues that the obselisk was placed so that the shadow of the obelisk would fall on the Ara Pacis, the altar of Peace, on Augustus’s birthday. There’s also a connection to the Mausoleum Augusti, his burial place, so that the whole area was a reflection of Augustus’s imposition of cosmic order. Heslin puts Buchner’s ideas as part of a long series of reconstructions dating back to the 17th century.
That was explained to me when I was part of a trip to Rome, and the explanation made no sense. If the shadow fell on the altar on Augustus’s birthday, which may have been on the 23rd of September, then the shadow must have passed over it at some time of day every day between the 20th of March and his birthday. The professor guiding us around the sites of ancient Rome was a professor of Ancient History, rather than Astronomy. A good choice, but it meant he couldn’t answer this query. It had been in the back of my mind to look more closely at the problem when I had time.
Why can’t it be a sundial?
Buchner may be one of a long line of people to propose the existence of a sundial on the Campus Martius — the about painting dates from the 19th century — but Heslin also sees he is the latest in a slightly shorter line of scholars debunking the existence of the sundial. He cites Angelo Maria Bandini as writing the definitive refutation of the sundial. In 1750! There are two closely connected questions. Why can’t it be a sundial, and if it can’t why has the idea that it is persisted for so long?
One reason the reconstruction fails is that the sundial would not work is hinted at in the painting above. If you look, you can see that the shadow gets lighter as it falls further from the base of the obelisk. This is accurate. The longer shadows get, the more diffuse they become. The German physicist Schütz has calculated the height of the obselisk from the calibration of a meridian line found in the cellar of a house on the Via di Campo Marzio. This shows that the shadow could never have reached as far as the Ara Pacis.
Additional evidence tends to evaporate on close inspection. Heslin goes through the arguments Schütz has made against the existence of a sundial. Buchner’s inscriptions seem to come from entirely different places to where Buchner claims. Heslin is scathing of Buchner’s work. He comments on Buchner’s interpretation of text to bolster a claim of an even more massive horologium:
Buchner’s solution is to dismiss these inconvenient parts of the passage as a ‘fantasy’. The real fantasy is this ‘wind-rose’ with its vast circular pavement, which was an attempt to salvage the erroneous claim that the sundial was found in San Lorenzo in the face of Schütz’s demonstration that the shadow of the obelisk could not usefully have reached that far. The tragedy is that this pure fiction is now enshrined in such fundamental reference work as the Lexicon Topographicum Urbis Romae.Heslin 2007:12
It doesn’t really get more brutal than that amongst ancient historians, though the comprehensive take-down of Buchner’s work by Heslin shows that it’s not said lightly.
If the paper were simply a debunking of the sundial, then that would be useful. However, Heslin goes on to put forward a better explanation which makes sense archaeologically, astronomically and historically.
If it’s not a sundial what is it?
There is archaeological evidience of a meridian line. The passage of Pliny above describes a meridian line. Heslin’s proposal sounds odd because it’s blindingly obvious. The obelisk, he argues, is a meridian.
A meridian is subtly different to a sundial. A sundial tells you the time of day. A meridian tells you the time of year. It’s one long line running north-south. At midday the sun is due south in Rome. When the sun is high in summer, the midday shadow is short. When the sun is low in winter, the midday shadow it casts is longer. If you look to see how long the shadow is when it’s aligned directly over the meridian line, then you can tell what day of the year it is. The archaeological excavations have revealed a line that runs directly north-south. One it are inscriptions, as an example on one side is ΤΑΥΡ[ΟΣ] and [ΛΕ]ΟΝ. Taurus and Leo, marking where the Sun was when it cast its shadow. The other inscriptions are about the time of year, not the time of day. But why set up a meridian rather than a sundial?
Julius Caesar reorganised the Roman calendar. Unfortunately there was a mix up with his instructions. Caesar said to insert a leap year every four years. Unfortunately the priests counted inclusively, so they were inserting leap years like this:
* Year One — leap year
* Year Two — standard year
* Year Three — standard year
* Year Four — fourth year so must be a leap year
Augustus had to make calendrical reforms of his own, because this is a leap year every three rather than four years. Mucking about with the calendar is not something you do on a whim. The usual explanation for calendars is that they’re for knowing when to plant crops. This isn’t likely. Farmers were able to harvest with the older inaccurate calendar. Calendars are more about social and religious order, which is easily confused with natural order. By fixing the calendar Augustus was making peace with the heavens, and that’s something worth marking. The meridian was a graphic illustration of the order Augustus had brought to the heavens, just as the Ara Pacis was propaganda to show how he had brought peace to earth. Heslin (2007:14–6) puts the two monuments in context of their position alongside the Via Flaminia, one of the main roads out of Rome.
It’s a cliché to say that history books will need to be re-written, but in this case Heslin has exposed some shockingly basic flaws in scholarship, not just in Buchner’s work but also in the work of many other scholars who have cited Schütz without engaging with the criticisms, or else simply ignored any criticisms altogether. Hopefully by exposing the flimsy nature of Buchner’s evidence, and providing a plausible alternative, Heslin’s work will have more impact that those who have tried previously to debunk the sundial.
Heslin, P. (2007). Augustus, Domitian and the So-called Horologium Augusti. Journal of Roman Studies, 97, 1–20.