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Ok here's the thread recap of my workshop with the Boston area high schoolers today. I talked about three Roman time keeping devices, and then had them make replicas to see how they worked.
I prepared these materials in advance and also made some examples ahead of time. The three devices were:
1) The 'Ham of Portici' (before 79 CE)
2) The 'Horologium' Augusti (10 BCE)
3) The parapegma from the Baths of Trajan (4th c. CE)
The 'Ham of Portici' is a portable, prosciutto-shaped sundial found at Herculaneum. In 2017 I heard Christopher Parslow give a paper on it at the #AIASCS. Here's @pompei79's thread from then: . This image of the ham is from @isawnyu:…
In Rome, the length of an hour depended on the time of year: shorter hours in winter and longer hours in summer. Here's a helpful infographic of the changing hours in 8 CE, by Darekk2 - Own work, CC BY-SA 4.0,…
The curved grid of the prosciutto sundial reflects the fact that Roman hours vary in length over the course of the year. Also reflected in the serpentine arrangement of the months:

June May April March Feb Jan
July August Sept Oct Nov Dec

In 2017, Christopher Parslow at the #AIASCS demonstrated how the gnomon would cast a shadow on the sundial at different times of day on a 3D printed replica. Here's an article about the sundial and the 3D printing by Christopher Chenier…
And here are two participants in the workshop today comparing their own clay replicas of the ham sundial under the “sun”, and discussing the need for a thin gnomon.
The second device I discussed is the so-called "Horologium" Augusti. In 10 BCE, Augustus brought a granite obelisk from Heliopolis to Rome and set it up in the Campus Martius, next to the Ara Pacis and nearby the Mausoleum. The obelisk kept time, but exactly how is debated.
This Egyptian obelisk, the "Obelisk of Montecitorio”, was originally made in the 6th c. BCE for Psammetichus II. When it was set up in the Campus Martius in Rome in 10 BCE, it received a Latin inscription dedicattng it to the Sun.
An influential theory put forward by Edmund Buchner in the 70s and 80s suggested the obelisk was part of a sundial, and that on Augustus' birthday (Sept. 23) this sundial cast a shadow pointing directly at Ara Pacis, connecting Augustus' cosmic power with his projected self-image
In 2007, Peter Heslin argued, based on a passage of Pliny, that the obelisk was not a sundial but a device to measure the solar meridian, which could be used to scientificially determine when the longest and shortest days of the year were (the solstice)
Julius Caesar had aligned the Roman civic calendar with the seasons - if Augustus' obelisk was a device to measure the solar meridian, it was a way to make sure that the civic calendar would not get out of step again.
1979 excavations in Campus Martius revealed a layer of travertine marble with Greek bronze letters relating to the zodiac, which Pliny had previously described. But the marble is from a later archaeological layer than the Ara Pacis, so appears not to be Augustan (Heslin 2007: 6)
Angelo Bandini's 1750 commentary on the obelisk (you can read online:…) was charmingly illustrated by James Stuart in these pull-out pages. I used Stuart's illustrations as the template for the student replicas, printing them on "granite"-coloured paper
Here are a couple of obelisks recreated by the students today, under the test of the "sun" (Augustus' obelisk was topped by a bronze globe).
The final device we discussed is an astrological parapegma from the 4th c. CE (discussed by Lehoux 2007: 168-170: ). This is the image of a lunar calendar: pegs (πῆγμα) are inserted in holes next to (παρά) the relevant information.
At the top, 7 gods representing the days of the week: Saturn (fragmentary), Sol, Luna, Mars, Mercury, Jupiter (fragmentary), Venus. In the centre: the 12 figures of the zodiac. Left: days 1-15, right: days 16-30 of lunar month.
Left: my recreation of the parapegma, made out of paper. I put a peg in "Thursday" (Jupiter) and Sagittarius. Right: one of the student participants made a replica out of clay.
If anyone is interested in doing a workshop like this, they're more than welcome to use my handouts. Here's the instructions for the replica-making, which asks the students to consider the object as they are remaking it. Full pdf here:…
I made the templates by printing the following images on coloured card. Here's the pdf:…
ok, bye! thanks for tuning in ;)
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