Already in the 3rd century AD, Dio Cassius wrote that «The Pantheon, due to its vaulted roof, resembles the celestial vault».
In 1991, the Italian astronomer Aldo Tavolaro discovered that the Pantheon was a large spherical sundial that indicated the seasons: «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».
One of these illuminations occurs on April 21, which was the Dies Natalis of the city of Rome.
The light illuminates the large bronze portal for fifteen consecutive days, from April 15 to 29. 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).
This and other spectacular illuminations such as the Arch and the Square of Light are explained in detail in the book by Marina De Franceschini and Giuseppe Veneziano: «Pantheon Architettura e Luce», with splendid unpublished photographs.
The illumination meant that it was the right day and the right time for religious rites, which could be presided over by the Emperor Hadrian in the guise of Pontifex Maximus, the highest Roman priestly office.
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.