Alun Salt mentioned this on Twitter and I finally have time to explore it a bit … A piece in New Scientist relates Robert Hannah’s suggestion that the Pantheon served as a sundial of some sort. Here’s an excerpt:
When Robert Hannah of the University of Otago in Dunedin, New Zealand, visited the Pantheon in 2005, researching for a book (see “Review: Time in Antiquity by Robert Hannah”), he realised that the Pantheon may have been more than just a temple. During the six months of winter, the light of the noon sun traces a path across the inside of the domed roof. During summer, with the sun higher in the sky, the shaft shines onto the lower walls and floor. At the two equinoxes, in March and September, the sunlight coming in through the hole strikes the junction between the roof and wall, above the Pantheon’s grand northern doorway (pictured). A grille above the door allows a sliver of light through to the front courtyard – the only moment in the year that it sees sunlight if its main doors are closed (see diagram).
Hannah reckons this is no coincidence. A hollowed-out hemisphere with a hole in the top was a type of sundial used in Roman times, albeit on a much smaller scale, to show the time of year. While the Pantheon’s dome is quite flat on the outside, it forms a perfect hemisphere inside. “This is quite a deliberate design feature,” says Hannah.
I tried to find a photo of one of these hollowed-out sundials, but Google is being very weird at the moment, but what I’d like to figure out is whether we can go beyond an ‘hour of the day’ idea to a full blown calendar idea. I think the interior of the Pantheon is much modified from Roman times and one could see the ceiling and walls being usefully used. Indeed, Cassius Dio (53.27) suggests:
2 Also he completed the building called the Pantheon. It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens.
If by ‘the heavens’ Dio is referring to actual decoration within the vaulted roof and those decorations were ‘accurate’, the dome could conceivably be used as a calendar, marking the sun’s position in relation to the zodiac or whatever, no?