Truncated icosahedron

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Truncated icosahedron
Truncated icosahedron
(Click here for rotating model)
Type Archimedean solid
Uniform polyhedron
Elements F = 32, E = 90, V = 60 (χ = 2)
Faces by sides 12{5}+20{6}
Schläfli symbols t{3,5}
t0,1{3,5}
Wythoff symbol 2 5 | 3
Coxeter diagram CDel node.pngCDel 5.pngCDel node 1.pngCDel 3.pngCDel node 1.png
Symmetry group Ih, H3, [5,3], (*532), order 120
Rotation group I, [5,3]+, (532), order 60
Dihedral Angle 6-6:138.189685°
6-5:142.62°
References U25, C27, W9
Properties Semiregular convex
Truncated icosahedron.png
Colored faces
Truncated icosahedron
5.6.6
(Vertex figure)
Pentakisdodecahedron.jpg
Pentakis dodecahedron
(dual polyhedron)
Truncated icosahedron flat-2.svg
Net

In geometry, the truncated icosahedron is an Archimedean solid, one of 13 convex isogonal nonprismatic solids whose faces are two or more types of regular polygons.

It has 12 regular pentagonal faces, 20 regular hexagonal faces, 60 vertices and 90 edges.

It is the Goldberg polyhedron GV(1,1), containing pentagonal and hexagonal faces.

This geometry is associated with footballs (soccer balls) typically patterned with white hexagons and black pentagons. Geodesic domes are often based on this structure. And it also corresponds to the geometry of the "Bucky Ball" (Carbon-60, or C60) molecule.

Construction[edit]

This polyhedron can be constructed from an icosahedron with the 12 vertices truncated (cut off) such that one third of each edge is cut off at each of both ends. This creates 12 new pentagon faces, and leaves the original 20 triangle faces as regular hexagons. Thus the length of the edges is one third of that of the original edges.

Icosahedron.png
Icosahedron

Cartesian coordinates[edit]

Cartesian coordinates for the vertices of a truncated icosahedron centered at the origin are all even permutations of:

(0, ±1, ±3φ)
(±2, ±(1+2φ), ±φ)
(±1, ±(2+φ), ±2φ)

where φ = (1 + √5) / 2 is the golden mean. Using φ2 = φ + 1 one verifies that all vertices are on a sphere, centered at the origin, with the radius squared equal to 9φ + 10. The edges have length 2.[1]

Orthogonal projections[edit]

The truncated icosahedron has five special orthogonal projections, centered, on a vertex, on two types of edges, and two types of faces: hexagonal and pentagonal. The last two correspond to the A2 and H2 Coxeter planes.

Orthogonal projections
Centered by Vertex Edge
5-6
Edge
6-6
Face
Hexagon
Face
Pentagon
Image Dodecahedron t12 v.png Dodecahedron t12 e56.png Dodecahedron t12 e66.png Icosahedron t01 A2.png Icosahedron t01 H3.png
Projective
symmetry
[2] [2] [2] [6] [10]

Dimensions[edit]

Mutually orthogonal golden rectangles drawn into the original icosahedron (before cut off)

If the edge length of a truncated icosahedron is a, the radius of a circumscribed sphere (one that touches the truncated icosahedron at all vertices) is:

r_u = \frac{a}{2} \sqrt{1 + 9\varphi^2} = \frac{a}{4} \sqrt{58 +18\sqrt{5}} \approx 2.47801866  \cdot a

where φ is the golden ratio.

This result is easy to get by using one of the three orthogonal golden rectangles drawn into the original icosahedron (before cut off) as the starting point for our considerations. The angle between the segments joining the center and the vertices connected by shared edge (calculated on the basis of this construction) is approx. 23.281446°.

Area and volume[edit]

The area A and the volume V of the truncated icosahedron of edge length a are:

\begin{align}
A & = \left ( 20 \cdot \frac32\sqrt{3} + 12 \cdot \frac54\sqrt{ 1 + \frac{2}{\sqrt{5}}} \right ) a^2 \approx 72.607253a^2 \\
V & = \frac{1}{4} \left(125+43\sqrt{5}\right) a^3 \approx 55.2877308a^3. \\
\end{align}

Geometric relations[edit]

The truncated icosahedron easily verifies the Euler characteristic:

32 + 60 − 90 = 2.

With unit edges, the surface area is (rounded) 21 for the pentagons and 52 for the hexagons, together 73 (see areas of regular polygons).

Applications[edit]

The truncated icosahedron (left) compared to a football.
Fullerene C60 molecule
Truncated icosahedral radome on a weather station

The balls used in association football and team handball are perhaps the best-known example of a spherical polyhedron analog to the truncated icosahedron, found in everyday life.[2] The ball comprises the same pattern of regular pentagons and regular hexagons, but it is more spherical due to the pressure of the air inside and the elasticity of the ball. This ball type was introduced to the World Cup in 1970 (starting in 2006, this iconic design has been superseded by alternative patterns).

Geodesic domes are typically based on this geometry with example structures found across the world, popularized by Buckminster Fuller.

A variation of the icosahedron was used as the basis of the honeycomb wheels (made from a polycast material) used by the Pontiac Motor Division between 1971 to 1976 on its Trans Am and Grand Prix.

This shape was also the configuration of the lenses used for focusing the explosive shock waves of the detonators in both the gadget and Fat Man atomic bombs.[3]

The truncated icosahedron can also be described as a model of the Buckminsterfullerene (fullerene) (C60), or "buckyball," molecule, an allotrope of elemental carbon, discovered in 1985. The diameter of the football and the fullerene molecule are 22 cm and ca. 1 nm, respectively, hence the size ratio is 220,000,000:1.

Truncated icosahedra in the arts[edit]

A truncated icosahedron with "solid edges" is a drawing by Lucas Pacioli illustrating The Divine Proportion.

Related polyhedra[edit]

Family of uniform icosahedral polyhedra
Symmetry: [5,3], (*532) [5,3]+, (532)
CDel node 1.pngCDel 5.pngCDel node.pngCDel 3.pngCDel node.png CDel node 1.pngCDel 5.pngCDel node 1.pngCDel 3.pngCDel node.png CDel node.pngCDel 5.pngCDel node 1.pngCDel 3.pngCDel node.png CDel node.pngCDel 5.pngCDel node 1.pngCDel 3.pngCDel node 1.png CDel node.pngCDel 5.pngCDel node.pngCDel 3.pngCDel node 1.png CDel node 1.pngCDel 5.pngCDel node.pngCDel 3.pngCDel node 1.png CDel node 1.pngCDel 5.pngCDel node 1.pngCDel 3.pngCDel node 1.png CDel node h.pngCDel 5.pngCDel node h.pngCDel 3.pngCDel node h.png
Uniform polyhedron-53-t0.png Uniform polyhedron-53-t01.png Uniform polyhedron-53-t1.png Uniform polyhedron-53-t12.png Uniform polyhedron-53-t2.png Uniform polyhedron-53-t02.png Uniform polyhedron-53-t012.png Uniform polyhedron-53-s012.png
{5,3} t{5,3} r{5,3} 2t{5,3}=t{3,5} 2r{5,3}={3,5} rr{5,3} tr{5,3} sr{5,3}
Duals to uniform polyhedra
CDel node f1.pngCDel 5.pngCDel node.pngCDel 3.pngCDel node.png CDel node f1.pngCDel 5.pngCDel node f1.pngCDel 3.pngCDel node.png CDel node.pngCDel 5.pngCDel node f1.pngCDel 3.pngCDel node.png CDel node.pngCDel 5.pngCDel node f1.pngCDel 3.pngCDel node f1.png CDel node.pngCDel 5.pngCDel node.pngCDel 3.pngCDel node f1.png CDel node f1.pngCDel 5.pngCDel node.pngCDel 3.pngCDel node f1.png CDel node f1.pngCDel 5.pngCDel node f1.pngCDel 3.pngCDel node f1.png CDel node fh.pngCDel 5.pngCDel node fh.pngCDel 3.pngCDel node fh.png
Icosahedron.svg Triakisicosahedron.jpg Rhombictriacontahedron.svg Pentakisdodecahedron.jpg POV-Ray-Dodecahedron.svg Deltoidalhexecontahedron.jpg Disdyakistriacontahedron.jpg Pentagonalhexecontahedronccw.jpg
V5.5.5 V3.10.10 V3.5.3.5 V5.6.6 V3.3.3.3.3 V3.4.5.4 V4.6.10 V3.3.3.3.5
Dimensional family of truncated polyhedra and tilings: n.6.6
Symmetry
*n42
[n,3]
Spherical Euclidean Compact hyperbolic Paracompact
*232
[2,3]
D3h
*332
[3,3]
Td
*432
[4,3]
Oh
*532
[5,3]
Ih
*632
[6,3]
P6m
*732
[7,3]
 
*832
[8,3]...
 
*∞32
[∞,3]
 
Order 12 24 48 120
Truncated
figures
Hexagonal dihedron.png
2.6.6
Uniform tiling 332-t12.png
3.6.6
Uniform tiling 432-t12.png
4.6.6
Uniform tiling 532-t12.png
5.6.6
Uniform tiling 63-t12.png
6.6.6
Uniform tiling 73-t12.png
7.6.6
Uniform tiling 83-t12.png
8.6.6
H2 tiling 23i-6.png
∞.6.6
Coxeter
Schläfli
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 2.pngCDel node.png
t{3,2}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 3.pngCDel node.png
t{3,3}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 4.pngCDel node.png
t{3,4}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 5.pngCDel node.png
t{3,5}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 6.pngCDel node.png
t{3,6}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 7.pngCDel node.png
t{3,7}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel 8.pngCDel node.png
t{3,8}
CDel node 1.pngCDel 3.pngCDel node 1.pngCDel infin.pngCDel node.png
t{3,∞}
Uniform dual figures
n-kis
figures
Hexagonal hosohedron.png
V2.6.6
Triakistetrahedron.jpg
V3.6.6
Tetrakishexahedron.jpg
V4.6.6
Pentakisdodecahedron.jpg
V5.6.6
Uniform tiling 63-t2.png
V6.6.6
Order3 heptakis heptagonal til.png
V7.6.6
Uniform dual tiling 433-t012.png
V8.6.6
H2checkers 33i.png
V∞.6.6
Coxeter CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 2.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 3.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 4.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 5.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 6.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 7.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel 8.pngCDel node.png CDel node f1.pngCDel 3.pngCDel node f1.pngCDel infin.pngCDel node.png

These uniform star-polyhedra, and one icosahedral stellation have nonuniform truncated icosahedra convex hulls:

Nonuniform truncated icosahedron.png
Nonuniform
truncated icosahedron
2 5 | 3
Great truncated dodecahedron.png
U37
2 5/2 | 5
Great dodecicosidodecahedron.png
U61
5/2 3 | 5/3
Uniform great rhombicosidodecahedron.png
U67
5/3 3 | 2
Great rhombidodecahedron.png
U73
2 5/3 (3/2 5/4)
Complete icosahedron ortho stella.png
Complete stellation
Rhombidodecadodecahedron convex hull.png
Nonuniform
truncated icosahedron
2 5 | 3
Rhombidodecadodecahedron.png
U38
5/2 5 | 2
Icosidodecadodecahedron.png
U44
5/3 5 | 3
Rhombicosahedron.png
U56
2 3 (5/4 5/2) |
Small snub icosicosidodecahedron convex hull.png
Nonuniform
truncated icosahedron
2 5 | 3
Small snub icosicosidodecahedron.png
U32
| 5/2 3 3

See also[edit]

Notes[edit]

  1. ^ Weisstein, Eric W., "Icosahedral group", MathWorld.
  2. ^ Kotschick, Dieter (2006). "The Topology and Combinatorics of Soccer Balls". American Scientist 94 (4): 350–357. 
  3. ^ Rhodes, Richard (1996). Dark Sun: The Making of the Hydrogen Bomb. Touchstone Books. p. 195. ISBN 0-684-82414-0. 

References[edit]

  • Williams, Robert (1979). The Geometrical Foundation of Natural Structure: A Source Book of Design. Dover Publications, Inc. ISBN 0-486-23729-X.  (Section 3-9)
  • Cromwell, P. (1997). Polyhedra. United Kingdom: Cambridge. pp. 79–86 Archimedean solids. ISBN 0-521-55432-2. 

External links[edit]