# Rhombic triacontahedron

In geometry, the rhombic triacontahedron, sometimes simply called the triacontahedron as it is the most common thirty-faced polyhedron, is a convex polyhedron with 30 rhombic faces. It has 60 edges and 32 vertices of two types. It is a Catalan solid, and the dual polyhedron of the icosidodecahedron. It is a zonohedron.

 A face of the rhombic triacontahedron. The lengths of the diagonals are in the golden ratio.

Rhombic triacontahedron

TypeCatalan solid
Coxeter diagram
Conway notationjD
Face typeV3.5.3.5
rhombus
Faces30
Edges60
Vertices32
Vertices by type20{3}+12{5}
Symmetry groupIh, H3, [5,3], (*532)
Rotation groupI, [5,3]+, (532)
Dihedral angle144°
Propertiesconvex, face-transitive isohedral, isotoxal, zonohedron

Icosidodecahedron
(dual polyhedron)

Net

The ratio of the long diagonal to the short diagonal of each face is exactly equal to the golden ratio, φ, so that the acute angles on each face measure 2 tan−1(1/φ) = tan−1(2), or approximately 63.43°. A rhombus so obtained is called a golden rhombus.

Being the dual of an Archimedean solid, the rhombic triacontahedron is face-transitive, meaning the symmetry group of the solid acts transitively on the set of faces. This means that for any two faces, A and B, there is a rotation or reflection of the solid that leaves it occupying the same region of space while moving face A to face B.

The rhombic triacontahedron is somewhat special in being one of the nine edge-transitive convex polyhedra, the others being the five Platonic solids, the cuboctahedron, the icosidodecahedron, and the rhombic dodecahedron.

The rhombic triacontahedron is also interesting in that its vertices include the arrangement of four Platonic solids. It contains ten tetrahedra, five cubes, an icosahedron and a dodecahedron. The centers of the faces contain five octahedra.

It can be made from a truncated octahedron by dividing the hexagonal faces into 3 rhombi:

## Dimensions

If the edge length of a rhombic triacontahedron is a, surface area, volume, the radius of an inscribed sphere (tangent to each of the rhombic triacontahedron's faces) and midradius, which touches the middle of each edge are:

{\begin{aligned}S&=12{\sqrt {5}}\,a^{2}&&\approx 26.8328a^{2}\\V&=4{\sqrt {5+2{\sqrt {5}}}}\,a^{3}&&\approx 12.3107a^{3}\\r_{\mathrm {i} }&={\frac {\varphi ^{2}}{\sqrt {1+\varphi ^{2}}}}\,a={\sqrt {1+{\frac {2}{\sqrt {5}}}}}\,a&&\approx 1.37638a\\r_{\mathrm {m} }&=\left(1+{\frac {1}{\sqrt {5}}}\right)\,a&&\approx 1.44721a\end{aligned}} where φ is the golden ratio.

The insphere is tangent to the faces at their face centroids. Short diagonals belong only to the edges of the inscribed regular dodecahedron, while long diagonals are included only in edges of the inscribed icosahedron.

## Dissection

The rhombic triacontahedron can be dissected into 20 golden rhombohedra: 10 acute ones and 10 obtuse ones.

1010

Acute form

Obtuse form

## Orthogonal projections

The rhombic triacontahedron has four symmetry positions, two centered on vertices, one mid-face, and one mid-edge. Embedded in projection "10" are the "fat" rhombus and "skinny" rhombus which tile together to produce the non-periodic tessellation often referred to as Penrose tiling.

 Projectivesymmetry Image Dualimage    

### Stellations

The rhombic triacontahedron has 227 fully supported stellations. Another stellation of the Rhombic triacontaedron is the compound of 5 octahedrons. The total number of stellations of the rhombic triacontahedron is 358,833,097.

This polyhedron is a part of a sequence of rhombic polyhedra and tilings with [n,3] Coxeter group symmetry. The cube can be seen as a rhombic hexahedron where the rhombi are also rectangles.

### 6-cube

The rhombic triacontahedron forms a 32 vertex convex hull of one projection of a 6-cube to three dimensions.

 The 3D basis vectors [u,v,w] are: u = (1, φ, 0, -1, φ, 0) v = (φ, 0, 1, φ, 0, -1) w = (0, 1, φ, 0, -1, φ) Shown with inner edges hidden20 of 32 interior vertices form a dodecahedron, and the remaining 12 form a icosahedron.

## Uses

Danish designer Holger Strøm used the rhombic triacontahedron as a basis for the design of his buildable lamp IQ-light (IQ for "Interlocking Quadrilaterals").

Woodworker Jane Kostick builds boxes in the shape of a rhombic triacontahedron. The simple construction is based on the less than obvious relationship between the rhombic triacontahedron and the cube.

Roger von Oech's "Ball of Whacks" comes in the shape of a rhombic triacontahedron.

The rhombic triacontahedron is used as the "d30" thirty-sided die, sometimes useful in some roleplaying games or other places.

Christopher Bird, co-author of The Secret Life of Plants wrote an article for New Age Journal in May, 1975, popularizing the dual icosahedron and dodecahedron as "the crystalline structure of the Earth," a model of the "Earth (telluric) energy Grid." The EarthStar Globe by Bill Becker and Bethe A. Hagens purports to show "the natural geometry of the Earth, and the geometric relationship between sacred places such as the Great Pyramid, the Bermuda Triangle, and Easter Island." It is printed as a rhombic triacontahedron, on 30 diamonds, and folds up into a globe.