Fig. 11. Root-3 geometric proportions in the Jefferson Rotunda. Thomas Jefferson, University of Virginia: Rotunda, South Elevation, 1819. Image courtesy of the Thomas Jefferson Papers, Special Collections, University of Virginia Library (N-328, K No. 8). Geometric overlay: Rachel Fletcher.
The Jefferson Rotunda, South Elevation: Root-3 Proportions ( Fig. 11 ). As in the Pantheon, a circle traces the exterior surface of the Rotunda dome. In fact, Jefferson draws such a circle, dotted in ink ( Fig. 2 ). A new circle of equal radius is drawn from the top of the dome. The result is a vesica piscis , with vertical and horizontal axes in 1:sqrt(3) ratio. The horizontal axis locates the base of the dome, which spans 120°“. A notation by Jefferson specifies that half the dome's surface spans "60°“. [ 13 ]
A square is drawn about the circle. Its base locates the baseline of the Rotunda. In addition, the top of the dome locates one apex of an equilateral triangle. The remaining two apexes touch the right and left sides of the square, while locating the floor level of the portico. The half-side of the equilateral triangle and its altitude are equal in length to the axes of the vesica piscis , in 1:sqrt(3) ratio.
Fletcher, R. (2003). An American Vision of Harmony: Geometric Proportions in Thomas Jefferson's Rotunda at the University of Virginia. Nexus Network Journal. Architecture and Mathematics Online, 5(2).
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