The Pantheon's Oculus

The Pantheons’s best known feature is probably its Oculus, a large circular hole in the center of its roof. The Oculus challenged the Pantheon’s builders by making its large, domed roof unstable. As a result, there are several features of the Pantheon’s design that are meant to reduce the roof’s strain on the rest of the building. The rim of the Oculus is surrounded by a thicker layer of concrete than the rest of the roof, reinforcing it in the absence of a capstone. A circular grid of square panels is sunken into the inside of the roof. The panels are deeper near the top of the roof, reducing its weight.

The Oculus, along with the massive entrance, are the only sources of light in the Pantheon. During the day, the circle of light cast by the Oculus circles the Pantheon’s floor. It is believed that on certain significant dates the light from the Oculus would shine directly on the entrance.

Of course, the Oculus does let rain in during storms. To compensate for this, the floor of the Pantheon is slightly concave, and there are several small drains in its center. In the present day, whenever rain is forecasted the space beneath the Oculus is simply roped off.

The Pantheon’s dome, built in 125 AD, was the last unsupported dome built in Europe until the Florence cathedral’s dome, which was completed in 1436.