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[GIF] Off the End (Stereographic projections of rotating regular polygons)

Stereographic projections of rotating regular polygons

Off the End

Each horizontal row shows the stereographic projection to the line of (the vertices of) a rotating regular polygon. The middle row shows the image of the vertices of an equilateral triangle, the rows above and below show a square, the rows above and below that a regular pentagon, etc. The speeds are all chosen so that one vertex passes through the south pole of the unit circle every $\frac{4}{5}$ of a second, which obviously means the triangle is spinning much faster than the regular 60-gon that shows up in the first and last rows. Here's an unoptimized animation showing all the rotating polygons to get a sense of their relative speeds:

Rotating regular polygons

Of course, this uses stereographic projection:

Stereo[p_] := p[[;; -2]]/(1 + p[[-1]]);

The actual image is set up as a GraphicsGrid; note that offset just a small irrational number to ensure that none of the polygon vertices pass through the north pole (where stereographic projection is undefined) at any of the time steps.

With[{bg = GrayLevel[.2], max = 60, offset = Sqrt[2.]/1000000},
 Manipulate[
    GraphicsGrid[
     Table[
      {Graphics[
        {White, PointSize[.005],
         Table[
            Point[Append[Stereo[ReIm[Exp[I (-2 ? t/n + ?/2 + 2 ? i/n)]]], 0]],
          {i, 1, n}]},
        Background -> bg, PlotRange -> {{-2, 2}, {-.0125, .0125}}]},
      {n, Join[Reverse[Range[4, max]], Range[3, max]]}],
     ImageSize -> 540, Background -> bg, Spacings -> 1, 
     PlotRangePadding -> None],
    {t, offset, 1 + offset - #, #}] &[1/40]
 ]

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POSTED BY: Moderation Team
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