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3D model of a 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, and 30 edges. It is represented by the Schläfli symbol {5,3}.

Relation to 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 . 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.[1]

The surface area and the volume of a regular dodecahedron of edge length are:[2]

The following Cartesian coordinates define the 20 vertices of a regular dodecahedron centered at the origin and suitably scaled and oriented:[3]

If the edge length of a regular dodecahedron is , the radius of a circumscribed sphere (one that touches the regular dodecahedron at all vertices), the radius of an inscribed sphere (tangent to each of the regular dodecahedron's faces), and the midradius (one that touches the middle of each edge) are:[4] Note that, given a regular dodecahedron of edge length one, is the radius of a circumscribing sphere about a cube of edge length , and is the apothem of a regular pentagon of edge length .

The dihedral angle of a regular dodecahedron between every two adjacent pentagonal faces is , approximately 116.565°.

As a Platonic solid

Regular dodecahedron painting by Johannes Kepler
Kepler's Platonic solid model of the Solar System

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.[5] 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—regular tetrahedron, cube, regular octahedron, and regular icosahedron—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."[6] Aristotle also postulated that the heavens were made of a fifth element, which he called aithêr (aether in Latin, ether in American English).[7]

Following its attribution with nature by Plato, Johannes Kepler in his Harmonices Mundi sketched each of the Platonic solids, one of them is a regular dodecahedron.[8] In his Mysterium Cosmographicum, Kepler also proposed the Solar System by using the Platonic solids setting into another one and separating them with six spheres resembling the six planets. The ordered solids started from the innermost to the outermost: regular octahedron, regular icosahedron, regular dodecahedron, regular tetrahedron, and cube.[9]

Many antiquity philosophers described the regular dodecahedron, including the rest of the Platonic solids. 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. Iamblichus states that Hippasus, a Pythagorean, perished in the sea, because he boasted that he first divulged "the sphere with the twelve pentagons".[10]

Relation to the regular icosahedron

The dual polyhedron of a dodecahedron is the regular icosahedron. One property of the dual polyhedron generally is that the original polyhedron and its dual share the same three-dimensional symmetry group. In the case of the regular dodecahedron, it has the same symmetry as the regular icosahedron, the icosahedral symmetry .[11]

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%).[12] The resulting of both spheres' volumes initially began from the problem by ancient Greeks, determining which of two shapes has a larger volume: an icosahedron inscribed in a sphere, or a dodecahedron inscribed in the same sphere. The problem was solved by Hero of Alexandria, Pappus of Alexandria, and Fibonacci, among others.[13] Apollonius of Perga discovered the curious result that the ratio of volumes of these two shapes is the same as the ratio of their surface areas.[14] Both volumes have formulas involving the golden ratio but are taken to different powers.[1]

Golden rectangle may also related to both regular icosahedron and regular dodecahedron. The regular icosahedron can be constructed by intersecting three golden rectangles perpendicularly, arranged in two-by-two orthogonal, and connecting each of the golden rectangle's vertices with a segment line. There are 12 regular icosahedron's vertices, considered as the center of 12 regular dodecahedron faces.[15]

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). As quoted by Coxeter et al. (1938),[16]

"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."

Configuration matrix

The configuration matrix is a matrix in which the rows and columns correspond to the elements of a polyhedron as in the vertices, edges, and faces. The diagonal of a matrix denotes the number of each element that appears in a polyhedron, whereas the non-diagonal of a matrix denotes the number of the column's elements that occur in or at the row's element. The regular dodecahedron can be represented in the following matrix:[17][18]

Other related geometric objects

The regular dodecahedron can be interpreted as a truncated trapezohedra. It is the set of polyhedrons that can be constructed by truncating the two axial vertices of a trapezohedron. Here, the regular dodecahedron is constructed by truncating the pentagonal trapezohedron.

The regular dodecahedron can be interpreted as the Goldberg polyhedron. It is a set of polyhedrons containing hexagonal and pentagonal faces. Other than two Platonic solids—tetrahedron and cube—the regular dodecahedron is the initial of Goldberg polyhedron construction, and the next polyhedron is resulted by truncating all of its edges, a process called chamfer. This process can be continuously repeated, resulting in more new Goldberg's polyhedrons. These polyhedrons are classified as the first class of a Goldberg polyhedron.[19]

The stellations of the regular dodecahedron make up three of the four Kepler–Poinsot polyhedra. The first stellation of a regular dodecahedron is constructed by attaching its layer with pentagonal pyramids, forming a small stellated dodecahedron. The second stellation is by attaching the small stellated dodecahedron with wedges, forming a great dodecahedron. The third stellation is by attaching the great dodecahedron with the sharp triangular pyramids, forming a great stellated dodecahedron.[20]

Appearances

In visual arts

Roman dodecahedron

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.

In toys and popular culture

In modern role-playing games, the regular dodecahedron is often used as a twelve-sided die, one of the more common polyhedral dice. 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 face of the regular dodecahedron describes the various facial expressions, swiveling 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
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 [21]

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.[22]

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.[23][21]

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,[24][25] and an optimal orientation on the sky for the model was estimated in 2008.[26]

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."

Dodecahedral graph

The Hamiltonian property of the dodecahedral and the mathematical game Icosian game

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.[27]

The dodecahedral graph is Hamiltonian, meaning there is a cycle containing all the vertices. The name of this property is named after William Rowan Hamilton, who invented a mathematical game known as the icosian game. The game's object was to find a Hamiltonian cycle along the edges of a dodecahedron.[28]

See also

References

  1. ^ a b Sutton, Daud (2002). Platonic & Archimedean Solids. Wooden Books. Bloomsbury Publishing USA. p. 55. ISBN 9780802713865.
  2. ^ Livio, Mario (2003) [2002]. The Golden Ratio: The Story of Phi, the World's Most Astonishing Number (First trade paperback ed.). New York City: Broadway Books. pp. 70–71. ISBN 0-7679-0816-3.
  3. ^ Weisstein, Eric W. "Icosahedral group". MathWorld.
  4. ^ Coxeter (1973) Table I(i), pp. 292–293. See the columns labeled , , and , Coxeter's notation for the circumradius, midradius, and inradius, respectively, also noting that Coxeter uses as the edge length (see p. 2).
  5. ^ Herrmann, Diane L.; Sally, Paul J. (2013). Number, Shape, & Symmetry: An Introduction to Number Theory, Geometry, and Group Theory. Taylor & Francis. p. 252. ISBN 978-1-4665-5464-1.
  6. ^ Plato, Timaeus, Jowett translation [line 1317–8]; the Greek word translated as delineation is diazographein, painting in semblance of life.
  7. ^ Wildberg, Christian (1988). John Philoponus' Criticism of Aristotle's Theory of Aether. Walter de Gruyter. pp. 11–12. ISBN 9783110104462.
  8. ^ Cromwell, Peter R. (1997). Polyhedra. Cambridge University Press. p. 57.
  9. ^ Livio (2003), p. 147.
  10. ^ Florian Cajori, A History of Mathematics (1893)
  11. ^ Erickson, Martin (2011). Beautiful Mathematics. Mathematical Association of America. p. 62. ISBN 978-1-61444-509-8.
  12. ^ Buker, W. E.; Eggleton, R. B. (1969). "The Platonic Solids (Solution to problem E2053)". American Mathematical Monthly. 76 (2): 192. doi:10.2307/2317282. JSTOR 2317282.
  13. ^ Herz-Fischler, Roger (2013). A Mathematical History of the Golden Number. Courier Dover Publications. pp. 138–140. ISBN 9780486152325.
  14. ^ Simmons, George F. (2007). Calculus Gems: Brief Lives and Memorable Mathematics. Mathematical Association of America. p. 50. ISBN 9780883855614.
  15. ^ Marar, Ton (2022). A Ludic Journey into Geometric Topology. p. 23. doi:10.1007/978-3-031-07442-4. ISBN 978-3-031-07442-4.
  16. ^ Coxeter, H.S.M.; du Val, Patrick; Flather, H. T.; Petrie, J. F. (1938). The Fifty-Nine Icosahedra. Vol. 6. University of Toronto Studies (Mathematical Series). p. 4.
  17. ^ Coxeter, H. S. M. (1973) [1948]. "§1.8 Configurations". Regular Polytopes (3rd ed.). New York: Dover Publications.
  18. ^ Coxeter, H. S. M. (1991). Regular Complex Polytopes (2nd ed.). Cambridge: Cambridge University Press. p. 117.
  19. ^ Hart, George (2012). "Goldberg Polyhedra". In Senechal, Marjorie (ed.). Shaping Space (2nd ed.). Springer. p. 127. doi:10.1007/978-0-387-92714-5_9. ISBN 978-0-387-92713-8.
  20. ^ Cromwell (1997), p. 265.
  21. ^ a b Kai Wu; Jonathan Nitschke (2023). "Systematic construction of progressively larger capsules from a fivefold linking pyrrole-based subcomponent". Nature Synthesis. doi:10.1038/s44160-023-00276-9.
  22. ^ Hagino, K., Onuma, R., Kawachi, M. and Horiguchi, T. (2013) "Discovery of an endosymbiotic nitrogen-fixing cyanobacterium UCYN-A in Braarudosphaera bigelowii (Prymnesiophyceae)". PLoS One, 8(12): e81749. doi:10.1371/journal.pone.0081749.
  23. ^ Dodecahedral Crystal Habit Archived 12 April 2009 at the Wayback Machine
  24. ^ Dumé, Belle (Oct 8, 2003). "Is The Universe A Dodecahedron?". PhysicsWorld. Archived from the original on 2012-04-25.
  25. ^ Luminet, Jean-Pierre; Jeff Weeks; Alain Riazuelo; Roland Lehoucq; Jean-Phillipe Uzan (2003-10-09). "Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background". Nature. 425 (6958): 593–5. arXiv:astro-ph/0310253. Bibcode:2003Natur.425..593L. doi:10.1038/nature01944. PMID 14534579. S2CID 4380713.
  26. ^ Roukema, Boudewijn; Zbigniew Buliński; Agnieszka Szaniewska; Nicolas E. Gaudin (2008). "A test of the Poincaré dodecahedral space topology hypothesis with the WMAP CMB data". Astronomy and Astrophysics. 482 (3): 747. arXiv:0801.0006. Bibcode:2008A&A...482..747L. doi:10.1051/0004-6361:20078777. S2CID 1616362.
  27. ^ Weisstein, Eric W. "Dodecahedral Graph". MathWorld.
  28. ^ Bondy, J. A.; Murty, U. S. R. (1976), Graph Theory with Applications, North Holland, p. 53, ISBN 0-444-19451-7

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