Regular Dodecahedron

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The regular dodecahedron, often simply called "the" dodecahedron, is the Platonic solid composed of 20 polyhedron vertices, 30 polyhedron edges, and 12 pentagonal faces, . It is illustrated above together with a wireframe version and a net that can be used for its construction.

The regular dodecahedron is also the uniform polyhedron with Maeder index 23 (Maeder 1997), Wenninger index 5 (Wenninger 1989), Coxeter index 26 (Coxeter et al. 1954), and Har'El index 28 (Har'El 1993). It is given by the Schläfli symbol and the Wythoff symbol .

A number of symmetric projections of the regular dodecahedron are illustrated above.

The regular dodecahedron is implemented in the Wolfram Language as Dodecahedron[] or UniformPolyhedron["Dodecahedron"], and precomputed properties are available as PolyhedronData["Dodecahedron"].

There are 43380 distinct nets for the regular dodecahedron, the same number as for the icosahedron (Bouzette and Vandamme, Hippenmeyer 1979, Buekenhout and Parker 1998). Questions of polyhedron coloring of the regular dodecahedron can be addressed using the Pólya enumeration theorem.

The image above shows an origami regular dodecahedron constructed using six dodecahedron units, each consisting of a single sheet of paper (Kasahara and Takahama 1987, pp. 86-87).

A dodecahedron appears as part of the staircase being ascending by alligator-like lizards in Escher's 1943 lithograph "Reptiles" (Bool et al. 1982, p. 284; Forty 2003, Plate 32). Two dodecahedra also appear as polyhedral "stars" in M. C. Escher's 1948 wood engraving "Stars" (Forty 2003, Plate 43). The IPV pod that transports Ellie Arroway (Jodi Foster) through a network of wormholes in the 1997 film Contact was enclosed in a dodecahedral framework.

A 40-foot high sculpture (Nath 1999) known as Eclipse is displayed in the Hyatt Regency Hotel in San Francisco. It was constructed by Charles Perry, and is composed of pieces of anodized aluminum tubes and assembled over a period of four months (Kraeuter 1999). The layered sculpture begins with a regular dodecahedron, but each face then rotates outward. At the midpoint of the rotation, it forms an icosidodecahedron. Then, as the 12 pentagons continue to rotate outward, it forms a small rhombicosidodecahedron.

Dodecahedra were known to the Greeks, and 90 models of dodecahedra with knobbed vertices have been found in a number of archaeological excavations in Europe dating from the Gallo-Roman period in locations ranging from military camps to public bath houses to treasure chests (Schuur).

The dodecahedron has the icosahedral group of symmetries. The connectivity of the vertices is given by the dodecahedral graph. There are three dodecahedron stellations.

The regular dodecahedron is the convex hull of the cube 5-Compound, ditrigonal dodecadodecahedron, third dodecahedron stellation hull, great ditrigonal icosidodecahedron, great rhombic triacontahedron, great stellated dodecahedron, rhombic hexecontahedron, small ditrigonalI icosidodecahedron, tetrahedron 5-compound, and tetrahedron 10-compound.

The dual polyhedron of a dodecahedron with unit edge lengths is an icosahedron with edge lengths , where is the golden ratio. As a result, the centers of the faces of an icosahedron form a dodecahedron, and vice versa (Steinhaus 1999, pp. 199-201).

A plane perpendicular to a axis of a dodecahedron cuts the solid in a regular hexagonal cross section (Holden 1991, p. 27). A plane perpendicular to a axis of a dodecahedron cuts the solid in a regular decagonal cross section (Holden 1991, p. 24).

A cube can be constructed from the dodecahedron's vertices taken eight at a time (above left figure; Steinhaus 1999, pp. 198-199; Wells 1991). Five such cubes can be constructed, forming the cube 5-compound. In addition, joining the centers of the faces gives three mutually perpendicular golden rectangles (right figure; Wells 1991).

The short diagonals of the faces of the rhombic triacontahedron give the edges of a dodecahedron (Steinhaus 1999, pp. 209-210).

The following table gives polyhedra which can be constructed by augmentation of a dodecahedron by pyramids of given heights .

		result
		60-faced dimpled deltahedron
		pentakis dodecahedron
		60-faced star deltahedron
		small stellated dodecahedron

When the dodecahedron with edge length is oriented with two opposite faces parallel to the -plane, the vertices of the top and bottom faces lie at and the other polyhedron vertices lie at , where is the golden ratio. The explicit coordinates are

(1)

(2)

with , 1, ..., 4, where is the golden ratio.

Eight dodecahedra can be place in a closed ring, as illustrated above (Kabai 2002, pp. 177-178).

The polyhedron vertices of a dodecahedron can be given in a simple form for a dodecahedron of side length by (0, , ), (, 0, ), (, , 0), and (, , ).

For a dodecahedron of unit edge length , the circumradius and inradius of a pentagonal face are

			(3)
			(4)

The sagitta is then given by

(5)

Now consider the following figure.

Using the Pythagorean theorem on the figure then gives

			(6)
			(7)
			(8)

Equation (8) can be written

(9)

Solving (6), (7), and (9) simultaneously gives

			(10)
			(11)
			(12)

The inradius of the regular dodecahedron is then given by

(13)

so

(14)

and solving for gives

(15)

Now,

(16)

so the circumradius is

(17)

The midradius is given by

(18)

so

(19)

The dihedral angle is

(20)

and the Dehn invariant for a unit regular dodecahedron is

			(21)
			(22)

where the first expression uses the basis of Conway et al. (1999).

The area of a single face is the area of a pentagon of unit edge length

(23)

so the surface area is 12 times this value, namely

(24)

The volume of the dodecahedron can be computed by summing the volume of the 12 constituent pentagonal pyramids,

(25)

Apollonius showed that for an icosahedron and a dodecahedron with the same inradius,

(26)

where is the volume and the surface area, with the actual ratio being

(27)