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Amy C. Edmondson
A Fuller Explanation
Chapter 14, Cosmic Railroad Tracks: Great Circles
pages 213 through 218

Least Common Denominator

The spherical icosahedron divides the surface of a sphere into the greatest number of completely regular domains (with both equal arc lengths and equal surface angles). We can therefore subdivide one of these symmetrical triangles to find the least common denomlnator of surface unity—in much the same way we isolated A and B modules. Each triangle is split into six equal parts by perpendicular bisectors, to obtain a final non-symmetrically—divisible unit (Fig. 14-9a). The result is 120 asymmetrical triangles (60 positive, 60 negative), the maximum number of equivalent domains on the surface of a sphere. These perpendicular bisectors are the icosahedron's fifteen great circles (Fig. 14-9b).

Thirty-one great circles
Fig. 14-8. Thirty-one great circles.
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      Imagine that some number of hypothetical creatures are to inhabit the surface of a large sphere and it is considered necessary that each one sit in the middle of an identical plot. The consequence of this stipulation is that no more than 120 creatures can fit on the sphere, no matter how large it is. This result is certainly counterintuitive, for assuming a large enough sphere, it seems that we should be able to accommodate as many creatures as we want. However, the unyielding laws of symmetry limit the population to the unexpectedly low number of 120.

Maximum of 120 equivalent domains: LCD
Fig. 14-9. Maximum of 120 equivalent domains: LCD.
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      We go back to the 31-great-circle diagram to observe the 120 triangles in context. These asymmetrical triangles are true LCD units; any one of them contains all the geometric information necessary to reconstruct the entire pattern. This is an important factor in the development of geodesic domes, as will be seen in the next chapter, which discusses the relationship of great circles to geodesic domes.

LCD: "Intertransformability"

The following exploration is similar to the transmutations of A and B modules and uncovers some of the same relationships; however, consistent with great-circle limitations, this study deals only with surface characteristics. Each symmetrical great-circle pattern has a least common denominator. For example, the spherical octahedron has eight equilateral faces, which can be split into six asymmetrical triangles, each one 1/6 of 1/8, or 1/48, of the whole surface. These triangles are LCD units, because they cannot be further subdivided to yield equivalent shapes.

      To isolate the LCD of 25 great circles, we must take into consideration that the spherical VE has two types of faces. Therefore, the smallest unit that can be reproduced to generate the whole pattern requires 1/6 of a VE triangle joined to 1/8 of an adjacent square. In this way, both aspects of the VE pattern are incorporated into the LCD, and the result is an asymmetrical triangle that covers 1/48 of the sphere (Fig. 14-10).

LCD unit
Fig. 14-10. LCD unit.
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      By changing the boundaries, LCDs of a given spherical polyhedron often can be caused to make up the faces of various other polyhedra. Interesting transformations are found between the great circles of the VE and the icosahedron, for the shift from 25 to 31 is another result of the icosahedron's "out of phase" role in the "cosmic hierarchy."

LCD of 31 Great Circles

Each triangle of the icosahedron consists of six LCDs, for a total of 120 asymmetrical triangles. That much is straightforward, as are the first two transformations.

Polyhedrons revealed in the 31-great-circle pattern
Fig. 14-11. Revealed in the 31-great-circle pattern: (a) rhombic triacontahedron; (b) pentagonal dodecahedron; (c) octahedron; and (d) VE.
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      Instead of the standard groups of six LCDs making up icosahedron faces, we change the boundaries. Four of these units form the diamond face of the spherical rhombic triacontahedron, so that we have thirty groups of four triangles instead of twenty groups of six. Figure 14-11 a shows that each icosahedron edge is the long diagonal of one of the thirty diamonds; the planar rhombic triacontahedron is thus a "degenerately stellated" icosahedron.

      Secondly, we can recombine the 120 units into twelve groups of ten with each triangular unit radiating out from an icosahedron vertex, to highlight the pentagonal dodecahedron (Fig. 14-11b).

      Next, we discover a few asymmetrical transformations. A spherical triangle of fifteen LCD units, incorporating a complete icosahedron triangle radially framed (like a pinwheel) by nine extra LCDs, is one face of the spherical octahedron. With this observation, we isolate the octahedron face just by looking at the pattern, so it's worth checking the arithmetic: multiply fifteen units per face by eight faces to get 120, or the whole system. Figure 14-11c shows this skew relationship, again reminiscent of the jitterbug transformation. And lastly, we observe the edges of a spherical VE. At first this seems to present a contradiction, given the unsynchronized relationship of the 25- and 31-great-circle patterns. However, the VE's four great circles are included among the 31 icosahedral equators, although asymmetrically positioned with respect to its vertices (Fig. 14-11d). We now recall from Chapter 11 that it was possible for all the faces of an octahedron to be aligned with eight of the icosahedron's twenty (Fig. 11-5). This skew correspondance, which defined the S module, shows how a subset of four out of the ten circles generated by icosahedron faces will be correctly situated to create the spherical VE.

      The number of different polyhedra hiding within the 31 great circles reemphasizes the existence of significant relationships between the "out of phase" icosahedral family and the IVM group.

VE's 25 Great Circles

The intertransformability displayed by the VE's least common denominator is straightforward, in contrast to the skew relationships demonstrated above. Groups of four units create diamond faces exactly centered over each VE vertex, thereby defining the twelve faces of its dual, the rhombic dodecahedron (Fig. 14-12a). Six LCD triangles come together to create octahedron faces (Fig. 14-12b), while the cube's six squares each consists of eight asymmetrical units (Fig. 14-12c). And a spherical tetrahedron uses a dozen LCDs per face, distributing the 48 units among only four faces (Fig. 14-12d).

Polyhedrons revealed in 25 great-circle pattern
Fig. 14-12. Revealed in 25 great-circle pattern: (a) rhombic dodecahedron; (b) octahedron; (c) cube; and (d) tetrahedron.
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      This brief look at various regroupings of LCD units shows how great-circle diagrams provide a new way to classify certain polyhedral characteristics, and thereby discover shared symmetries between systems.

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