While he was alive, the Roman emperor acted as an intermediary between man and the gods, and in the role of Pontifex Maximus he controlled Time and therefore Power: political and religious power were one and the same.
The emperor Hadrian acted as an intermediary between men and the gods, and had to watch over and maintain the Pax Deorum, that is, the favorable protection of the divinity over the empire and over himself.
Hadrian could say he was so powerful that he could even command the course of the Sun, especially on certain days of the year when spectacular illuminations such as the Arc of Light occurred. They were a luminous signal that that was the right day and the right time for certain sacred rituals.
The idea was taken up again in 1966 by Kjeld De Fine Licht – who wrote one of the fundamental texts on the Pantheon: «The dome in ancient times was interpreted as an imitation of the celestial vault – and therefore the geometric shape of the Pantheon was created as an allusion to the Cosmos».
In 1976, Passuello and Dissegna were the first to connect the Pantheon to Archaeoastronomy: «The dome represents the sky or the celestial vault, since it seems to rotate infinitely around the central point, that is, the man who stands in this space».
The real turning point came in 1991, when the Italian astronomer Aldo Tavolaro discovered that the Pantheon functioned as a large seasonal sundial: «It is possible to determine the dates of the year by observing the position of the spot of light at astronomical noon [...] If it is winter, the oval of light never descends below the cornice, if it is summer it descends to illuminate the floor of the temple».
In the winter period, therefore, the circle of light always illuminates the dome above the cornice for a period of six months, which includes the Winter Solstice and is roughly between the two Equinoxes (from September 28 to March 17).
Starting from the Spring Equinox, the various illuminations (hierophanies) begin: Equinox, Dies Natalis of Rome and Summer Solstice.
Equinox
– On the days of the Equinox (March 21 and September 23) the circle of light illuminates the cornice at the base of the dome and filters through the grate above the portal. This illumination is visible from March 17 to 24 and from September 20 to 27, for a total of fifteen days.
Dies Natalis of Rome
- On April 21 (Dies Natalis of Rome) the light illuminates the large bronze portal for fifteen consecutive days, from April 15 to 29.54 The same illumination is also visible four months later, from August 13 to 27, days that are not linked to any particular holiday of the Roman Calendar (except August 15, Feriae Augusti).
Summer Solstice
On the Summer Solstice (June 21) a huge circle of light illuminates the floor almost in the center.55 This illumination is visible for over a month, from June 5 to July 7.
Arc of Light
The symbolic meaning of this extraordinary phenomenon, which presupposes very accurate astronomical calculations made 1900 years ago, is explained in detail in the book by