Much can be learnt about the purpose of the Pantheon by studying its history and its architecture.
It is said that Marcus Agrippa built the Pantheon in 27BC in honour of Emperor Augustus‘ divine self and that Augustus himself refused so prominent position: He didn’t want to make the mistake of Caesar and was careful to drive an image of a man at the service of the people.
The name Pantheon has a lot to say: the building is believed to have been dedicated to all the gods of Rome. As such the niches around the inside must have contained altars and/or statues of various deities. Agrippa’s and Augustus’ statues were probably placed somewhere in the vicinity, possibly just outside the door.
The Pantheon we now see was actually rebuilt several times and Emperor Hadrian is the one who rendered the perfection we know and love. Only the portico which bears Agrippa’s name is re-used and gives us an idea of how important Agrippa’s name had remained. So in a sense the Pantheon has also served as a sort of memorial.
Given the spherical conception of Hadrian’s Pantheon it is easily imaginable that it was dedicated to the planetary deities. The Orb was in fact a symbol of the world and universe and consequently doubled up as a symbol of the Emperor’s power over the world. Another detail which relate the Pantheon to the heavens is that it has 7 niches for altars and statues and at that time there were seven known planets.
Could it be that Agrippa’s Pantheon and Hadrian’s later Pantheon were dedicated to a different set of deities or that the later one restricted them to the planets? Can’t say, but at least the gods Mars and Venus would always have had a place of importance in the temple given that they were regarded as the ancestors from whom the Julio-Claudian line of Emperors from Caesar and Augustus through to Nero descended. They are also two of the planets known to the Romans.
The Oculus in the Roof
The “Oculus” hole at the top of the roof is a further element which leads us to deduce that a cosmic effect was being sought. The Oculus allows light (and rain!) in and when the sun is shining the shaft of light travels across the interior of the building, rather like being in a large scale observatory where the heavens are turned outside-in.
You might suggest that the Oculus is purely there as a solution for light, just like many other Roman houses. On the other hand, why include the Oculus at all if not to create a direct link with the sun? How can we say that? All other Roman temples before and since seemed to cope perfectly well without the Oculus solution. Even Hadrian himself designed the enormous temple to Venus and Rome without such a device. The only possible exception I know of is the so called “Temple of Mercury” at Baia, near Naples but I think that was actually a bit of thermal baths rather than a temple.
Conclusion: The Oculus had more reason to it than one of simple lighting. Furthermore this wouldn’t be the only occasion in history where a hole in the ceiling is used for planetary observations or as a sun-dial.
On the other hand, if we really try hard to relate the Pantheon to the heavens we will also note that when we look up at the dome we see the central ring of the Oculus, surrounded by 5 rings of coffers. These could be related to a heliocentric view of the cosmos with the sun in the centre and Jupiter, Saturn, Mars, Venus and Moon around it (not necessarily in that order). What about Earth? You’re standing on it. Could this have doubled up as some sort of a planetarium which allowed a more or less accurate prediction of planetary positions?