The icosahedron, having twenty equilateral triangles as faces.
Associated words: vicenary, vigesimal, vigesimation, icosahedron.
These give the solid angles of the tetrahedron, the octahedron and the icosahedron.
Within the earth's orbit inscribe an icosahedron: the sphere inscribed in it will be Venus's orbit.
Now, inscribe in the Earth an icosahedron, the circle inscribed in it will be Venus.
An icosahedron is a figure comprehended under twenty equal sides.
That for the tetrahedron has 12 for its order, for the cube (or octahedron) 24, and for the icosahedron (or dodecahedron) 60.
Then inscribe in the Earth an icosahedron; the circle described in it will be Venus.
Two of these generate the cube and the octahedron; five of these generate the dodecahedron and the icosahedron.
1560s, from Greek eikosahedron, neuter of eikosahedros, from eikosi "twenty" + -hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit" (see sedentary). Greek eikosi is from PIE *wikmti- "twenty," from *wi- "in half," hence "two" + (d)kmti-, from root *dekm- "ten" (see ten).