The dome of the Pantheon represents the celestial vault, and every day at noon the rays of the Sun enter from the Oculus creating a Circle of Light towards the portal.
In the days of the Equinox, the Circle of Light hits the Cornice at the base of the dome, and marks the change of season, that is, the beginning of Autumn (or Spring).
This illumination is visible for a week, from 20 to 27 September, or from 17 to 24 March.
This and other magic illuminations with their symbolic meaning are explained in detail with wonderful new pictures in the book of Marina De Franceschini «Pantheon. Architecture & Light» published by Rirella Editrice.