The wooden Dome of the rock Rotskoepel (end 7th century - Jeruzalem) with a perimeter of 20,44 m seems to be supported by an octogonal building, but it isn't. In rality it is supported by a ring of colums. The octogon is just built around it.

The most logical support of a spherical dome is a cylinder. But a cylinder has, apart of the use as independant building little constructive possibilities. In Classical Antiquity round buildings are used as temples and aphitheatres.[br]If you want to use spherical domes in rectangular buildings, you'll have to think of solutions to support the round form of a spherical dome.