This extraordinary temple, one of the main tourist sites of Rome, was completed in the first half of the second century c.e. The building itself survives almost intact and is undoubtedly one of the architectural wonders of the world. Its enormous dome, more than forty-three meters (142 feet) in diameter, remained the largest in the world for thirteen centuries. It is made of solid concrete and is near-perfectly hemispherical on the inside (although, for practical reasons, it is thinner and less dense toward the top), and supported upon masonry walls more than six meters (twenty feet) thick. At its apex is a circular hole nine meters (twenty-nine feet) across, so that the visitor feels at once enclosed and yet exposed to the elements. Though now largely empty, the building was once richly and colorfully decorated. The ceiling of the dome was stuccoed, and stones of different colors (porphyry, marble, and granite) were used in the walls and floor. We can imagine the numerous recesses filled with statues of the gods.
Although more recently consecrated as a Catholic church, the temple was originally dedicated to the sun and stars. An intriguing question is the extent to which religious and astronomical symbolism was reflected in the architecture. Proportionality evidently mattered, in that the height is precisely the same as the width; in other words, if one were to imagine the dome being extended into a complete sphere it would just touch the ground in the middle. A more controversial matter is whether sun-and-shadow effects were deliberately incorporated. It is certainly true that as the sun moves through the sky at different times of the year and day, natural light entering through the apex creates eye-catching patterns on the walls and floor. To what extent particular effects were intentional and orchestrated to occur at significant times remains largely a matter of speculation.
