The Pantheon's Dome

From The Wall Street Journal:

Seen from its north-facing front, the concrete-and-brick Pantheon consists of a pedimented entrance porch, a domed rotunda and a boxlike intermediate structure joining them. Their forms—triangle, hemisphere and rectangle—announce the underlying theme of pure geometry.

Inside, rising from a circle-and-square-patterned floor, the hemispherical coffered dome rests on a drum. The drum's bottom level is ringed with tabernacles alternating with recessed spaces screened with columns, its upper one with blind windows and framed marble panels.

But it is what's overhead that draws the gasps: the largest masonry dome ever built—142 feet in diameter and weighing five thousand tons—it is the paterfamilias of every structure like it erected since. At the top is one of the most famous features in architecture, the oculus. It focuses a circle of light into the Pantheon that, tracking the transit of the sun, passes slowly across the interior surfaces as the day progresses. This moving disc—glowing, silent, inexorable—transforms the Pantheon from bricks-and-mortar house of worship into an almost living thing.

Understandably, given its scale and spread, discussions of the Pantheon have tended to focus on engineering—what it took to erect it and keep it standing. To minimize its weight, the dome thins as it rises, starting at about 20 feet thick at the bottom and tapering to only about four feet at the oculus. Lighter aggregates were mixed with the cement as the dome rose.

The 28 radiating lines of coffers help, too. There has been much speculation over whether there is any symbolic significance to their number. That seems unlikely. The dome was the most critical part of the enterprise—one mistake and you've got rubble—so structural rather than numerological considerations surely drove their design.

If this hunch is correct, some formidable calculations would have been required. The architect would first have had to determine by what cubic footage the mass of the dome needed to be reduced; then figure out the size, shape and number of voids necessary to meet that requirement; finally, arrange them in a way that was uniform, well-ordered and pleasing to the eye. His task makes the creative feats of today's architects, with their CAD (computer-aided design) programs, seem a tad anticlimactic.

Inside, the dome springs from midway between floor and oculus. Outside, the springing point is significantly higher, the rotunda having been extended upward to buttress the dome. Two levels of heavy vaults concealed inside the rotunda walls transfer the dome's thrust down onto eight massive supporting piers.

Herein lies the essence of the Roman revolution in architecture. A Greek temple is structurally transparent, each element—column, capital, entablature—is visible and its role in forming and supporting the whole self-evident. But the Romans' epic architectural ambitions demanded new and more complex methods of construction. So we see the onset of a kind of illusionism, where the architectural effects and the means used to realize them travel separate paths. Today such sleights of hand are all around us in the modern skyscraper's glass-curtain wall.

But the Pantheon is about more than engineering. It is about space—architectural space as a conduit to spiritual space.

The Pantheon is the greatest interior in Western architecture, one where space is nearly as palpable as the forms that contain it—what isn't there is as important as what is. This effect derives in part from the perfection of its proportions. As William L. MacDonald writes in his 1976 book on the building (still the indispensable guide to the subject), the Pantheon is a sphere within a cube. Continue the curvature of the dome downward, and you get an orb whose bottommost surface kisses the floor. Then raise four vertical planes at the cardinal points of the rotunda, capping them with a horizontal one brushing the oculus, and, with the floor, they'll give you a container cube for the sphere.

It is also a function of the way voids, rather than, as is customary, masses, are used to articulate and embellish the interior. The tabernacles are the only projections; everything else recedes: the column-screened recessions between them, above those the blind windows, above those the coffers, and topping them all the ultimate void—the oculus.

Because of the vertical alignment of these elements, the eye is naturally drawn upward, and as it moves, we notice that the forms become simpler, more elemental. We trace a passage that gradually removes us from the specific, worldly realm below to the most abstract, universal shape of all. The oculus is many things. It is the Pantheon's basic design module. It is an act of consummate architectural audacity. Most of all, however, it is a portal to the heavens. (Read entire article.)

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