Regular dodecahedron

A regular dodecahedron or pentagonal dodecahedron is a dodecahedron that is regular, which is composed of 12 regular pentagonal faces, three meeting at each vertex. It is one of the five Platonic solids. It has 12 faces, 20 vertices, 30 edges, and 160 diagonals (60 face diagonals, 100 space diagonals). It is represented by the Schläfli symbol {5,3}.

Regular dodecahedron
Type	Platonic solid
Faces	12 regular pentagons
Edges	30
Vertices	20
Symmetry group	icosahedral symmetry $\mathrm {I} _{\mathrm {h} }$
Dihedral angle (degrees)	$2\arctan(\phi )$
Properties	convex, regular
Net

3D model of a regular dodecahedron

Crystal structure of Co20L12 dodecahedron reported by Kai Wu, Jonathan Nitschke and co-workers at University of Cambridge in Nat. Synth. 2023, DOI:10.1038/s44160-023-00276-9

Relation between the golden ratio

The golden ratio is the ratio between two numbers equal to the ratio of their sum to the larger of the two quantities. It is one of two roots of a polynomial, expressed as ${\textstyle \phi ={\frac {1+{\sqrt {5}}}{2}}\approx 1.618}$. The golden ratio can be applied to the regular dodecahedron's metric properties, as well as to construct the regular dodecahedron. A regular dodecahedron is a polyhedron with 12 pentagonal faces, 30 edges, and 20 vertices.

The surface area $A$ and the volume $V$ of a regular dodecahedron of edge length $a$ are: $$A={\frac {15\phi }{\sqrt {3-\phi }}},\qquad V={\frac {5\phi ^{3}}{6-2\phi }}.$$

The following Cartesian coordinates define the 20 vertices of a regular dodecahedron centered at the origin and suitably scaled and oriented: $${\begin{aligned}(\pm 1,\pm 1,\pm 1),&\qquad (0,\pm \phi ,\pm 1/\phi ),\\(\pm 1/\phi ,0,\pm \phi ),&\qquad (\pm \phi ,\pm 1/\phi ,0).\end{aligned}}$$

If the edge length of a regular dodecahedron is $a$, the radius of a circumscribed sphere $r_{u}$ (one that touches the regular dodecahedron at all vertices), the radius of an inscribed sphere $r_{i}$ (tangent to each of the regular dodecahedron's faces), and the midradius $r_{m}$ (one that touches the middle of each edge) are:(sequence A179296 in the OEIS) $${\begin{aligned}r_{u}&={\frac {\phi {\sqrt {3}}}{2}}a\approx 1.401a,\\r_{i}&={\frac {\phi ^{2}}{2{\sqrt {3-\phi }}}}a\approx 1.114a,\\r_{m}&={\frac {\phi ^{2}}{2}}a\approx 1.309a.\end{aligned}}$$ Note that, given a regular dodecahedron of edge length one, $r_{u}$ is the radius of a circumscribing sphere about a cube of edge length $\phi$, and $r_{i}$ is the apothem of a regular pentagon of edge length $\phi$.

The dihedral angle of a regular dodecahedron between every two adjacent pentagonal faces is $2\arctan(\phi )$, approximately 116.565°

As a Platonic solid

Regular dodecahedron painting by Johannes Kepler

The regular dodecahedron is one of the Platonic solids, a set of polyhedrons in which the faces are regular polygons that are congruent and the same number of faces meet at a vertex. This set of polyhedrons is named after Plato. In Theaetetus, a dialogue of Plato, Plato hypothesized that the classical elements were made of the five uniform regular solids. Plato described the regular dodecahedron, obscurely remarked, "...the god used [it] for arranging the constellations on the whole heaven". Timaeus, as a personage of Plato's dialogue, associates the other four Platonic solids with the four classical elements, adding that there is a fifth solid pattern which, though commonly associated with the regular dodecahedron, is never directly mentioned as such; "this God used in the delineation of the universe." Aristotle also postulated that the heavens were made of a fifth element, which he called aithêr (aether in Latin, ether in American English).

As a configuration

This configuration matrix represents the dodecahedron. The rows and columns correspond to vertices, edges, and faces. The diagonal numbers say how many of each element occur in the whole dodecahedron. The nondiagonal numbers say how many of the column's element occur in or at the row's element.

${\begin{bmatrix}{\begin{matrix}20&3&3\\2&30&2\\5&5&12\end{matrix}}\end{bmatrix}}$

Here is the configuration expanded with k-face elements and k-figures. The diagonal element counts are the ratio of the full Coxeter group H₃, order 120, divided by the order of the subgroup with mirror removal.

H3		k-face	fk	f0	f1	f2	k-fig	Notes
A2		( )	f0	20	3	3	{3}	H3/A2 = 120/6 = 20
A1A1		{ }	f1	2	30	2	{ }	H3/A1A1 = 120/4 = 30
H2		{5}	f2	5	5	12	( )	H3/H2 = 120/10 = 12

Geometric relations

The regular dodecahedron is the third in an infinite set of truncated trapezohedra which can be constructed by truncating the two axial vertices of a pentagonal trapezohedron.

The stellations of the regular dodecahedron make up three of the four Kepler–Poinsot polyhedra.

A rectified regular dodecahedron forms an icosidodecahedron.

The regular dodecahedron has icosahedral symmetry I_h, Coxeter group [5,3], order 120, with an abstract group structure of A₅ × Z₂.

Relation to the regular icosahedron

The dodecahedron and icosahedron are dual polyhedra. A regular dodecahedron has 12 faces and 20 vertices, whereas a regular icosahedron has 20 faces and 12 vertices. Both have 30 edges.

When a regular dodecahedron is inscribed in a sphere, it occupies more of the sphere's volume (66.49%) than an icosahedron inscribed in the same sphere (60.55%).

A regular dodecahedron with edge length 1 has more than three and a half times the volume of an icosahedron with the same length edges (7.663... compared with 2.181...), which ratio is approximately 3.51246117975, or in exact terms: 3/5(3ϕ + 1) or (1.8ϕ + 0.6).

Relation to the nested cube

A cube can be embedded within a regular dodecahedron, affixed to eight of its equidistant vertices, in five different positions. In fact, five cubes may overlap and interlock inside the regular dodecahedron to result in the compound of five cubes.

The ratio of the edge of a regular dodecahedron to the edge of a cube embedded inside such a regular dodecahedron is 1 : ϕ, or (ϕ − 1) : 1.

The ratio of a regular dodecahedron's volume to the volume of a cube embedded inside such a regular dodecahedron is 1 : 2/2 + ϕ, or 1 + ϕ/2 : 1, or (5 + √5) : 4.

For example, an embedded cube with a volume of 64 (and edge length of 4), will nest within a regular dodecahedron of volume 64 + 32ϕ (and edge length of 4ϕ − 4).

Thus, the difference in volume between the encompassing regular dodecahedron and the enclosed cube is always one half the volume of the cube times ϕ.

From these ratios are derived simple formulas for the volume of a regular dodecahedron with edge length a in terms of the golden mean:

V = (aϕ)³ · 1/4(5 + √5)

V = 1/4(14ϕ + 8)a³

Relation to the regular tetrahedron

Five tetrahedra inscribed in a dodecahedron. Five opposing tetrahedra (not shown) can also be inscribed.

As two opposing tetrahedra can be inscribed in a cube, and five cubes can be inscribed in a dodecahedron, ten tetrahedra in five cubes can be inscribed in a dodecahedron: two opposing sets of five, with each set covering all 20 vertices and each vertex in two tetrahedra (one from each set, but not the opposing pair).

Just as a tetrahedron can be inscribed in a cube, so a cube can be inscribed in a dodecahedron. By reciprocation, this leads to an octahedron circumscribed about an icosahedron. In fact, each of the twelve vertices of the icosahedron divides an edge of the octahedron according to the "golden section". Given the icosahedron, the circumscribed octahedron can be chosen in five ways, giving a compound of five octahedra, which comes under our definition of stellated icosahedron. (The reciprocal compound, of five cubes whose vertices belong to a dodecahedron, is a stellated triacontahedron.) Another stellated icosahedron can at once be deduced, by stellating each octahedron into a stella octangula, thus forming a compound of ten tetrahedra. Further, we can choose one tetrahedron from each stella octangula, so as to derive a compound of five tetrahedra, which still has all the rotation symmetry of the icosahedron (i.e. the icosahedral group), although it has lost the reflections. By reflecting this figure in any plane of symmetry of the icosahedron, we obtain the complementary set of five tetrahedra. These two sets of five tetrahedra are enantiomorphous, i.e. not directly congruent, but related like a pair of shoes. [Such] a figure which possesses no plane of symmetry (so that it is enantiomorphous to its mirror-image) is said to be chiral.

Relation to the golden rectangle

Golden rectangles of ratio (ϕ + 1) : 1 and ϕ : 1 also fit perfectly within a regular dodecahedron. In proportion to this golden rectangle, an enclosed cube's edge is ϕ, when the long length of the rectangle is ϕ + 1 (or ϕ²) and the short length is 1 (the edge shared with the regular dodecahedron).

In addition, the center of each face of the regular dodecahedron form three intersecting golden rectangles.

Relation to the 6-cube and rhombic triacontahedron

Projection of 6-demicube into regular dodecahedral envelope

It can be projected to 3D from the 6-dimensional 6-demicube using the same basis vectors that form the hull of the rhombic triacontahedron from the 6-cube. Shown here including the inner 12 vertices, which are not connected by the outer hull edges of 6D norm length √2, form a regular icosahedron.

The 3D projection basis vectors [u,v,w] used are:

u = (1, ϕ, 0, −1, ϕ, 0)

v = (ϕ, 0, 1, ϕ, 0, −1)

w = (0, 1, ϕ, 0, −1, ϕ)

History and uses

Roman dodecahedron

Omnidirectional sound source

Regular dodecahedral objects have found some practical applications, and have also played a role in the visual arts and in philosophy.

Iamblichus states that Hippasus, a Pythagorean, perished in the sea, because he boasted that he first divulged "the sphere with the twelve pentagons".

Theaetetus gave a mathematical description of all five and may have been responsible for the first known proof that no other convex regular polyhedra exist. Euclid completely mathematically described the Platonic solids in the Elements, the last book (Book XIII) of which is devoted to their properties. Propositions 13–17 in Book XIII describe the construction of the tetrahedron, octahedron, cube, icosahedron, and dodecahedron in that order. For each solid Euclid finds the ratio of the diameter of the circumscribed sphere to the edge length. In Proposition 18 he argues that there are no further convex regular polyhedra.

Regular dodecahedra have been used as dice and probably also as divinatory devices. During the Hellenistic era, small, hollow bronze Roman dodecahedra were made and have been found in various Roman ruins in Europe. Their purpose is not certain.

In 20th-century art, dodecahedra appear in the work of M. C. Escher, such as his lithographs Reptiles (1943) and Gravitation (1952). In Salvador Dalí's painting The Sacrament of the Last Supper (1955), the room is a hollow regular dodecahedron. Gerard Caris based his entire artistic oeuvre on the regular dodecahedron and the pentagon, which is presented as a new art movement coined as Pentagonism.

A climbing wall consisting of three dodecahedral pieces

In modern role-playing games, the regular dodecahedron is often used as a twelve-sided die, one of the more common polyhedral dice.

Immersive Media Company, a former Canadian digital imaging company, made the Dodeca 2360 camera, the world's first 360° full-motion camera which captures high-resolution video from every direction simultaneously at more than 100 million pixels per second or 30 frames per second. It is based on regular dodecahedron.

The Megaminx twisty puzzle, alongside its larger and smaller order analogues, is in the shape of a regular dodecahedron.

In the children's novel The Phantom Tollbooth, the regular dodecahedron appears as a character in the land of Mathematics. Each of his faces wears a different expression – e.g. happy, angry, sad – which he swivels to the front as required to match his mood.

In nature and supramolecules

The fossil record of the coccolithophore Braarudosphaera bigelowii goes back 100 million years

The faces of a Holmium–magnesium–zinc (Ho-Mg-Zn) quasicrystal are true regular pentagons

The fossil coccolithophore Braarudosphaera bigelowii (see figure), a unicellular coastal phytoplanktonic alga, has a calcium carbonate shell with a regular dodecahedral structure about 10 micrometers across.

Some quasicrystals and cages have dodecahedral shape (see figure). Some regular crystals such as garnet and diamond are also said to exhibit "dodecahedral" habit, but this statement actually refers to the rhombic dodecahedron shape.

Shape of the universe

Various models have been proposed for the global geometry of the universe. In addition to the primitive geometries, these proposals include the Poincaré dodecahedral space, a positively curved space consisting of a regular dodecahedron whose opposite faces correspond (with a small twist). This was proposed by Jean-Pierre Luminet and colleagues in 2003, and an optimal orientation on the sky for the model was estimated in 2008.

In Bertrand Russell's 1954 short story "The Mathematician's Nightmare: The Vision of Professor Squarepunt", the number 5 said: "I am the number of fingers on a hand. I make pentagons and pentagrams. And but for me dodecahedra could not exist; and, as everyone knows, the universe is a dodecahedron. So, but for me, there could be no universe."

Vertex arrangement

The regular dodecahedron shares its vertex arrangement with four nonconvex uniform polyhedra and three uniform polyhedron compounds.

Five cubes fit within, with their edges as diagonals of the regular dodecahedron's faces, and together these make up the regular polyhedral compound of five cubes. Since two tetrahedra can fit on alternate cube vertices, five and ten tetrahedra can also fit in a regular dodecahedron.

Great stellated dodecahedron	Small ditrigonal icosidodecahedron	Ditrigonal dodecadodecahedron	Great ditrigonal icosidodecahedron
Compound of five cubes	Compound of five tetrahedra	Compound of ten tetrahedra

Stellations

The 3 stellations of the regular dodecahedron are all regular (nonconvex) polyhedra: (Kepler–Poinsot polyhedra)

	0	1	2	3
Stellation	Regular dodecahedron	Small stellated dodecahedron	Great dodecahedron	Great stellated dodecahedron
Facet diagram

Dodecahedral graph

Regular dodecahedron graph
A Hamiltonian cycle in a dodecahedron.
Vertices	20
Edges	30
Radius	5
Diameter	5
Girth	5
Automorphisms	120 (A5 × Z2)
Chromatic number	3
Properties	Hamiltonian, regular, symmetric, distance-regular, distance-transitive, 3-vertex-connected, planar graph
Table of graphs and parameters

The skeleton of the dodecahedron (the vertices and edges) form a graph. It is one of 5 Platonic graphs, each a skeleton of its Platonic solid.

This graph can also be constructed as the generalized Petersen graph G(10,2) where the vertices of a decagon are connected to those of two pentagons, one pentagon connected to odd vertices of the decagon and the other pentagon connected to the even vertices. Geometrically, this can be visualized as the 10-vertex equatorial belt of the dodecahedron connected to the two 5-vertex polar regions, one on each side.

The high degree of symmetry of the polygon is replicated in the properties of this graph, which is distance-transitive, distance-regular, and symmetric. The automorphism group has order 120. The vertices can be colored with 3 colors, as can the edges, and the diameter is 5.

The dodecahedral graph is Hamiltonian – there is a cycle containing all the vertices. Indeed, this name derives from a mathematical game invented in 1857 by William Rowan Hamilton, the icosian game. The game's object was to find a Hamiltonian cycle along the edges of a dodecahedron.

Orthogonal projection

See also

• 120-cell, a regular polychoron (4D polytope whose surface consists of 120 dodecahedral cells)
• Braarudosphaera bigelowii − A dodecahedron shaped coccolithophore (a unicellular phytoplankton algae).
• Dodecahedrane (molecule)
• Pentakis dodecahedron
• Snub dodecahedron
• Truncated dodecahedron

External links

• Weisstein, Eric W. "Regular Dodecahedron". MathWorld.
• Klitzing, Richard. "3D convex uniform polyhedra o3o5x – doe".
• Editable printable net of a dodecahedron with interactive 3D view
• The Uniform Polyhedra
• Origami Polyhedra – Models made with Modular Origami
• Dodecahedron – 3-d model that works in your browser
• Virtual Reality Polyhedra The Encyclopedia of Polyhedra
  • VRML#Regular dodecahedron
• K.J.M. MacLean, A Geometric Analysis of the Five Platonic Solids and Other Semi-Regular Polyhedra
• Dodecahedron 3D Visualization
• Stella: Polyhedron Navigator: Software used to create some of the images on this page.
• How to make a dodecahedron from a Styrofoam cube
• The Greek, Indian, and Chinese Elements – Seven Element Theory