One of Rome’s major monuments has gone missing

Ancient Rome (Detail)

Blogging on Peer-Reviewed ResearchThe 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?

Campus Martius

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?

Meridian line in Rome
A Meridian line in Rome. Photo from Wikipedia.

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.

Peer Reviewed Heslin, P. (2007). Augustus, Domitian and the So-called Horologium Augusti. Journal of Roman Studies, 97, 1–20.


When he's not tired, ill or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.

3 Responses

  1. archaeozoo says:

    The sounds like one of those occa­sions when a ‘story’ has simply been re-told time and again without check­ing the accur­acy of it.

    The altern­at­ive solu­tion is interesting.

  1. January 24, 2008

    […] by the Ara Pacis, is not a sun­dial at all. It’s a meridian. A what? Go read the whole post at Archaeoastronomy. This entry was writ­ten by Jeremy, pos­ted on 24/1/2008 at 7:19 am, filed under Geeky, Glory. […]

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