Everyone passes the Pantheon on the way from here to there in Rome. People arrange to meet in front of it, they sit on its steps, they see it from the cafes and fountain of the Piazza della Rotonda, and they gaze at the inscription on the architrave which reads “M AGRIPPA L F COS TERTIUM FECIT” or “Marcus Agrippa, son of Lucius, third time consul, made this temple.”

When I lived in Rome in 1999, the Pantheon had closed for Jubilee renovations, but when I returned and lived just behind this “temple of all the gods” for a summer, the scaffolding had come down to reveal from both outside and in, the best preserved and most intact monument of ancient Rome, survivor of the perils of nineteen centuries.

Originally constructed in 27 B.C. by Agrippa, the Pantheon was rebuilt by the Emperor Hadrian in 125 A.D. to reflect the terrestrial and cosmic order. Its great portico is supported by eight monolithic granite columns with white marble capitals and bases. The rectangular portico connects to the rotunda by a massive brickwork structure and screens the vast dome within so completely that it is only from the inside that the dome’s scale and soaring beauty can be perceived.

One enters through gigantic bronze doors – the originals – and remembers suddenly that there were once veneers of precious marbles within, pure gold tiles on the roof, and that the bronze doors, weighing twenty tons each, were themselves once covered with plates of beaten gold.

Inside the temple is a cavernous architectural space with a ponderous cylindrical wall supporting the cupola, or dome. The base was constructed entirely out of poured concrete without the support of vaults, arches, or ribs -- a feat still puzzling to archaeologists and architects. The walls of the drum supporting the dome are seventeen feet thick, and the cupola, with its 142-foot diameter, is the largest vault ever constructed in masonry – St. Peter’s dome, modeled on that of the Pantheon, is only 138 feet in diameter. The mathematical perfection of the Pantheon is underscored by its symmetry here: the width and the height of the dome are equal. There is a progressive narrowing of the five coffered rows of the cupola’s interior, so that the eye is drawn to the center. And up, up, up to a feature called the oculus.

From Wikipedia, the free encyclopedia

Ah, the oculus. This is a hole in the top of the Pantheon. Nothing on the exterior, nothing short of an aerial view from a helicopter, could prepare a visitor for this thirty-foot hole. In a building of perfect harmony and balance, the hole opens up the vault to the sky and becomes, simply stated, divine.

When I was struck by the celestial, I hadn’t yet opened my guidebook, preferring first to experience, and then to read and augment. And like most visitors, I hardly imagined that the top was actually open. So I walked around the Pantheon, and as I looked up, a shaft of blinding white light made me gasp. It made a geometric disk as it hit the rows of coffers with a dizzying power, and electrified a disk on the floor, accomplishing an instantaneous transmogrification from the architectural to the spiritual.

In her Memoirs of Hadrian, Marguerite Yourcenar speaks of “this open and secret temple, conceived as a sundial. The hours were to circle the center of its carefully polished pavement where the disk of the day was supposed to rest like a golden buckler; there the rain would make a limpid pool from which prayer could transpire like smoke toward the void where we place the gods.”

Put less poetically, the thirty-foot hole in the ceiling performs like a sundial to mark the passing of the hours and the dates of equinox and solstice. The hole is the building’s only source of light. Or air. And it symbolizes a union between earth and sky that allows human prayer to ascend to the heavens unimpeded.

To experience the sundial effect, I made it a point to return to the Pantheon at varying times of day, to watch a white or a golden light pounce on the floor or walls, or simply to observe dark clouds moving across the oculus. I came back often in a light rain and again in heavy rains to experience the phenomenon of rain in the temple by sound and sense, and I had to search assiduously for the drainholes, integrated so ingeniously into the pattern of the floor, and then watched the water as it was carried away into drain channels made in 27 B.C. by Agrippa. And I often think that I will have to live in Rome in winter someday for the pure joy of a snowfall, descending, luminous, from the oculus into the temple like the Holy Spirit itself…