SPRING OF LIGHT IN THE PANTHEON
From March 17 to 24, at noon.
During the winter, in the months between the Autumn Equinox (September 23) and the Spring Equinox (March 21), every day at noon a Circle of Light appears above the portal, but always remains above the cornice that separates the large dome from the walls of the Pantheon.
When Spring arrives, the Circle of Light moves downwards and illuminates the cornice, as was discovered in 1991 by the Italian astronomer Aldo Tavolaro. He was the first to understand the archaeoastronomical function of the dome, which symbolically represented the celestial vault.
On the days of the Spring Equinox (from March 17 to 24) and of the Autumn Equinox (September 20 to 27) the Circle of Light created by the oculus illumnates the cornice: «The image of the Sun is placed on the cornice for a few moments, just as the Sun crosses the celestial equator towards that date», Tavolaro wrote.
This means that the Pantheon functioned as a giant sundial; the height of the Circle of Light inside it varies with the passing of the Seasons, and therefore it is a calendar indicator.
This and other magical illuminations that occur throughout the year inside the Pantheon are illustrated with magnificent photographs in the book by Marina De Franceschini: «Pantheon Architecture & Light» (also in English language) which explains in detail their symbolic meaning.