Why probably did brunelleschi use nine circles of horizontal ribs for the dome?

The dome refers to the Santa Maria del Fiore in Florence, Italy.

Because the architect, Brunelleschi did not leave any structural drawings or design for the dome, many engineers and architects have studied the dome as a feat of engineering accomplishment for many years.

A plausible explanation for the structural design of the dome is that although it is an octagonal structure, it was designed as if it were circular, self-supporting structure.
The nine circles provided a skeletal structure on top of the inner dome for tensile and compressive strength, while simultaneously preventing the outer dome from falling inward.

Both countries had to aprove annexation.