two ellipses = circle

dx = N[2/36];
a = 0.5;
c1 = ColorData["DarkRainbow"][0];
c2 = ColorData["DarkRainbow"][1];
Manipulate[
 With[{t = 0.25 + 0.67 (0.5 + 0.5 Sin[2 Pi \[Tau]])},
  Graphics[
   Translate[
    {
     {
      c1,
      Table[
       With[{y = Max[1 - t, a Sqrt[1 - x^2]]},
        Line[{{x, y - a Sqrt[1 - x^2]}, {x, y + a Sqrt[1 - x^2]}}]
        ], {x, -1 + dx/2, 1, dx}]
      },
     {
      c2,
      Table[
       With[{y = Min[-1 + t, -a Sqrt[1 - x^2]]},
        Line[{{x, y - a Sqrt[1 - x^2]}, {x, y + a Sqrt[1 - x^2]}}]
        ], {x, -1 + dx/2, 1, dx}]
      }

     },
    {0, -t}
    ], PlotRange -> {{-1, 1}, {-1.25 - 0.75, 1.25}},
   ImagePadding -> 10]
  ],
 {\[Tau], 0, 1}]