From September 20th to 27th, you can admire the Autumn Equinox illuminations in the Pantheon, starting at 12:30 am, when a Circle of Light will illuminate the cornice above the portal.
Let's now explain their astronomical significance: the Equinoxes and Solstices are the most important astronomical events of the year, marking the change of Seasons.
The Winter Solstice, December 21st, marks the end of Autumn and the beginning of Winter, with the apparent death of Nature, which, for three months, sort of goes into hibernation. The days become increasingly shorter, colder, and darker. Snowfall, strong winds, and storms arrive.
A few days later, the New Year begins, which was celebrated and exorcised with special rituals very similar to our New Year's celebrations, to propitiate the rebirth of Nature and the arrival of a new cycle of Seasons.
For the ancients, Time was cyclical and not in constant flux as it is today. The Winter Solstice marked the beginning of a new cycle, with an ascending phase that culminates six months later on the Summer Solstice, before returning to the starting point with a descending phase.
The Spring Equinox, March 21st, marks the end of Winter and the arrival of Spring, with the rebirth of Nature, the blossoming of flowers, and the budding of crops. As the name suggests, day and night are of equal length: twelve hours of daylight and twelve hours of darkness at night. From that moment on, the days become increasingly longer and the nights shorter.
The Summer Solstice, June 21st, marks the end of Spring and the beginning of Summer. It is the time of Nature's greatest luxuriance: the crops are harvested and the fruits are gathered. The days are the longest of the year, the Sun is the hottest. From that date, the descending cycle begins, the days begin to shorten, and we move toward the end of the annual cycle.
The Autumn Equinox, September 23, mirrors the Spring Equinox. It marks the end of Summer and the beginning of Autumn, when it's time to harvest, sow, and store food for the winter. The days become shorter, the sun less intense, and rain and bad weather begin.
The Pantheon, as we explained several times in our posts, functioned like a giant hemispheric Sundial. Every day at solar noon (1:00 pm in summer time), the Sun enters through the dome's oculus and projects a Circle of Light toward the portal. This circle has different heights depending on the season, thus serving as a calendar indicator.
Particularly during the winter period between the two Equinoxes, the Circle of Light is visible high up on the dome, and is always above the cornice that marks the beginning of the dome itself.
On the days of the Equinoxes, that is, from September 20th to 27th or from March 17th to 24th, the Circle of Light is on the cornice. This illumination was discovered in 1991 by the Italian astronomer Aldo Tavolaro: he was the first to understand the archaeo-astronomical function of the dome, which symbolically represents the celestial vault.
Beginning on March 24th, the Circle of Light appears lower and lower, until it illuminates the portal on Rome's Dies Natalis, April 21st. During the summer, from June to mid July a large Circle of Light appears on the floor of the Pantheon.
The most extraordinary and significant luminous phenomenon of the Pantheon, however, is the Arc of Light, which occurs neither on the Solstices nor the Equinoxes, but only on April 6th, 7th, and 8th and September 4th, 5th, and 6th.
This illumination, together with with the Square of Light, had a very specific and fascinating symbolic meaning, which we explain in detail in our book "Pantheon. Architecture and Light," published by Rirella Editrice.