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I was playing with a rectangular prism 4x4x3 (almost a cube) and I was giving him a rotating movement with the hand over the table so I could see a visual effect I have seen many times in my life and now I remembered to ask about it.

So when I give to it an impulse and it start to rotate I can see very well defined smooth edges, well really a circular edge, with the actual vertex diffusal. In other words, like if it was a cylinder rotating with almost invisible vertex and edges.

I understand that we see the only constant points in the rotation move, this is the incircle (or in-crylinder) of the prism. The vertex only passes in discrete times over the same points so that is why it becomes diffusal.

However I want to know the mathematical/physical explanation of this.

Thank you in advance.