Sunlight has always been a symbol of divinity. In the Pantheon special illuminations take place which were a sacred signal: that was the right day and the right time to celebrate religious rites. They could be presided over by the emperor Hadrian in the guise of Pontifex Maximus, the highest Roman priestly office.

In winter, when the Sun is lower and the days are shorter, the Circle of Light appears high in the dome. On the days of the Equinox (March 21st and September 23rd) when Spring begins, the Circle of Light hits the cornice at the base of the dome.

In summer, when the sun is higher and the days are longer, an enormous circle of light illuminates the floor of the Pantheon almost in the centre. It is visible for one month, from 5 June to 7 July, and culminates in the days of the Summer Solstice (June 21st).

April 21st was a special day: the day of the foundation of Rome, that is, Dies Natalis. The portal of the Pantheon is illuminated by an oval of light that is seen twice a year: from 15 to 29 April and from 13 to 27 August, for a total of 30 days.

The most extraordinary and spectacular illumination, however, is the Arc and the Square of Light, which are visible for very few days a year: on 6-7-8 April and 4-5-6 September.
The Ac and the Square had a precise symbolic meaning, linked to the cult of four important deities: Diana, goddess of the Moon, Apollo god of the Sun, Cybele goddess Mother of all the gods and Jupiter Optimus Maximus, protector of the city of Rome.

The meaning of the illumination of the portal on April 21st, of the Arc and Square of Light and of all other luminous phenomena are explained in detail in our book «Pantheon. Architecture and Light».
It traces the thousand-year history of this masterpiece of Roman architecture, explains its building techniques and shows the illuminations with splendid unpublished photographs.

The book reconstructs the triumphal entrance in the Pantheon of Emperor Hadrian. He was semi-divine, so powerful that he could command the course of the Sun. He was illuminated by the Sun as by a theatrical spotlight, and was the Cosmokrator, ruler of the Universe.

The emperor was standing in the center of the Square of Light which was the center of the world and then passed under the Arch of Light, which framed him like an Arch of triumph, almost like a halo.

Also on April 21st, when he entered through the portal, the Sun illuminated him and dressed him with Light, because he was the intermediary between men and the gods.