# Spinning Dipyramids

I made these videos using my cell phone and a magnetic ball-and-stick polyhedron building system which my wife bought for me. It’s the sticks that have magnets in them, not the steel balls. First, a triangular dipyramid (n = 3). This is the simplest of the dipyramids.

Next, a square dipyramid, also known as an octahedron (n = 4).

Next, for n = 5, the pentagonal dipyramid.

If you limit yourself to dipyramids that have equilateral triangles for faces, that’s the complete set. Here’s what happens when you try n = 6 — the dipyramid has zero height, and collapses into a pair of isosceles trapezoids when lifted.

To get this to work, you’d need to use isosceles triangles, not equilateral ones. The same is true for n = 7 and greater numbers.

# Compound of Three Octagonal Dipyramids

This compound was created using Stella 4d, software you can try here.

# A Compound of Three Square Dipyramids

The reason I am not calling this a compound of three octahedra is that the faces of the dipyramids aren’t quite equilateral. They are, however, isosceles.

# More Polyhedra, Including Some Compounds, from the Stellation-Series of the Tetrakis Cube

The next one is a compound of eight off-center pyramids. By this point, I had gone so far into the stellation-series (a search I began when preparing the post before this one) that I had lost count.

This one is a compound of three short square-based dipyramids:

This one, according to Stella 4d, is a compound of three parts, but I can’t quite figure out what the parts are!

Here is another “mystery compound,” this one with two parts:

Stella 4d, which I used to make these, may be tried here.

## A Pulsating Compound of Three Octangular Dipyramids

### Image

Software credit: see http://www.software3d.com/Stella.php for the software used (Stella 4d) to make this image. A free trial download is available.

## Compound of Four Non-Regular Triangular Dipyramids

### Image

The only irregularity to be found here is that the faces of the dipyramids are isosceles, not equilateral.

Software credit: http://www.software3d.com/stella.php

## A Compound of Three Non-Regular Square Diyramids

### Image

The only irregularity to be found here is that the faces of the dipyramids are isosceles, not equilateral.

Software credit: see this website to try the program I used to make this rotating .gif image. It’s called Stella 4d: Polyhedron Navigator.