Triangular hebesphenorotunda

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Triangular hebesphenorotunda
Triangular hebesphenorotunda.png
Type Johnson
J91 - J92 - J1
Faces 13 triangles
3 squares
3 pentagons
1 hexagon
Edges 36
Vertices 18
Vertex configuration 3(33.5)
6(3.4.3.5)
3(3.5.3.5)
2.3(32.4.6)
Symmetry group C3v
Dual polyhedron -
Properties convex
Net
Johnson solid 92 net.png

In geometry, the triangular hebesphenorotunda is one of the Johnson solids (J92).

A Johnson solid is one of 92 strictly convex regular-faced polyhedra, but which is not uniform, i.e., not a Platonic solid, Archimedean solid, prism or antiprism. They are named by Norman Johnson who first enumerated the set in 1966.

It is one of the elementary Johnson solids, which do not arise from "cut and paste" manipulations of the Platonic and Archimedean solids. However, it does have a strong relationship to the icosidodecahedron, an Archimedean solid. Most evident is the cluster of three pentagons and four triangles on one side of the solid. If these faces are aligned with a congruent patch of faces on the icosidodecahedron, then the hexagonal face will lie in the plane midway between two opposing triangular faces of the icosidodecahedron.

The triangular hebesphenorotunda is the only Johnson solid with faces of 3, 4, 5 and 6 sides. The 92 Johnson solids were named and described by Norman Johnson in 1966.

Coordinates[edit]

The coordinates of the triangular hebesphenorotunda are:

  • The triangle opposite the hexagon:
\left(0,\ \frac{2}{\sqrt3},\ \frac{2\phi^2}{\sqrt3}\right), \left(\pm1,\ -\frac{1}{\sqrt3},\ \frac{2\phi^2}{\sqrt3}\right)
  • The bases of the triangles surrounding the previous triangle:
\left(\pm1,\ \frac{\phi^3}{\sqrt3},\ \frac{2\phi}{\sqrt3}\right), \left(\pm\phi^2,\ -\frac{1}{\phi\sqrt3},\ \frac{2\phi}{\sqrt3}\right), \left(\pm\phi,\ -\frac{\phi+2}{\sqrt3},\ \frac{2\phi}{\sqrt3}\right)
  • The tips of the pentagons opposite the first triangle:
\left(\pm\phi^2,\ \frac{\phi^2}{\sqrt3},\ \frac{2}{\sqrt3}\right), \left(0,\ -\frac{2\phi^2}{\sqrt3},\ \frac{2}{\sqrt3}\right)
  • The hexagon:
\left(\pm1,\ \pm\sqrt3,\ 0\right), \left(\pm2,\ 0,\ 0\right)

where \phi=\frac{1+\sqrt5}{2} is the Golden Ratio.

These coordinates produce a triangular hebesphenorotunda with edge length 2, resting on the XY plane and having its 3-fold axis of symmetry aligned to the Z-axis. A second, inverted, triangular hebesphenorotunda can be obtained by negating the second and third coordinates of each point. This second polyhedron will be joined to the first at their common hexagonal face, and the pair will inscribe an icosidodecahedron. If the hexagonal face is scaled by the Golden Ratio, then the convex hull of the result will be the entire icosidodecahedron.

External links[edit]