Square cupola explained

In geometry, the square cupola (sometimes called lesser dome) is a cupola with an octagonal base. In the case of all edges being equal in length, it is a Johnson solid, a convex polyhedron with regular faces.It can be used to construct many other polyhedrons, particularly other Johnson solids.

Properties

The square cupola has 4 triangles, 5 squares, and 1 octagon as their faces; the octagon is the base, and one of the squares is the top. If the edges are equal in length, the triangles and octagon become regular, and the edge length of the octagon is equal to the edge length of both triangles and squares. The dihedral angle between both square and triangle is approximately

, that between both triangle and octagon is , that between both square and octagon is precisely , and that between two adjacent squares is . A convex polyhedron in which all the faces are regular is a Johnson solid, and the square cupola is enumerated as , the fourth Johnson solid. Given that the edge length of , the surface area of a square cupola can be calculated by adding the area of all faces:$$A\; =\; \backslash left(7+2\backslash sqrt+\backslash sqrt\backslash right)a^2\; \backslash approx\; 11.560a^2.$$Its height , circumradius , and volume are: of order 8.

Related polyhedra and honeycombs

, gyroelongated square cupola , square orthobicupola , square gyrobicupola , elongated square gyrobicupola , and gyroelongated square bicupola .

The crossed square cupola is one of the nonconvex Johnson solid isomorphs, being topologically identical to the convex square cupola. It can be obtained as a slice of the nonconvex great rhombicuboctahedron or quasirhombicuboctahedron, analogously to how the square cupola may be obtained as a slice of the rhombicuboctahedron. As in all cupolae, the base polygon has twice as many edges and vertices as the top; in this case the base polygon is an octagram. It may be seen as a cupola with a retrograde square base, so that the squares and triangles connect across the bases in the opposite way to the square cupola, hence intersecting each other.

The square cupola is a component of several nonuniform space-filling lattices:

• with tetrahedra;
• with cubes and cuboctahedra; and
• with tetrahedra, square pyramids and various combinations of cubes, elongated square pyramids and elongated square bipyramids.^[1]

Notes and References

1. Web site: J4 honeycomb.