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The dome over the center of Hagia Sophia has since its construction been an object of fascination, as much for its design as for its size. To stand within the space it covers is awe-inspiring. Though not unprecedented in form, the support structure for this massive dome sets the building apart. The architectural challenge the church’s designers overcame arose from a desire to set a circular dome over a square structure, especially if and when the walls of that structure would feature sizable arches.

In a traditional four-cornered room, and one in which arches (spanning the side walls) converge at the corners, a vertical line from floor to ceiling defines the juncture point of any two walls. A circular dome set on top of such a four-cornered structure would rest upon the walls, but upon the walls’ center points, above the arches. Most likely resting, therefore, upon the weakest points of those arched walls. Yet, the dome would not receive direct support at the building’s corners, usually the building’s strongest points, because the perimeter of the dome’s base would sit away from the square corners.

The genius of Hagia Sophia’s immense dome was the incorporation of what are formally termed “pendentives.” This architectural feature can be seen in the photo at the top, in the triangle-shaped, semi-spherical corner panels that allow the four corner columns to support the essential load-bearing portions of the dome’s base. It is illustrated in the diagram below (with the letter “a” representing the pendentives).

Readers of my prior post, upon the Greek Cross plan for Hagia Sophia, will observe how this mosque-cathedral has a basilica or rectangular shape at the upper level, while yet featuring a Greek Cross-shaped floor plan (see below). This is due the absence of half or semi-domes extending from the north and south sides of the building as they observably do on both the west and east ends.

Instead of the presence of half or semi-domes below the great arches on the upper level of the north and south sides of Hagia Sophia we find what are called “tympanaum.” These are wall screens that are punctuated by windows (see below).

Subsequent mosque designs, such as the breathtaking Süleymaniye Mosque in Istanbul (see below), took the same design concept forward while emphasizing the overall square shape of the Greek Cross.

These are some of the ways in which Hagia Sophia’s design set the stage for later mosque architecture. Later mosque plans tended to incorporate half or semi-domes on all four sides of the central, full-dome covered portion of the prayer space. In turn, these plans, as well as those of their predecessors, continue to influence the design of Christian churches in ways that can enhance the whole congregation’s participation in liturgy. (For more on this point, please see my prior post, “The Beauty of Hagia Sophia, and the Greek Cross”).