Evolute of an ellipse
Evolute is the envelope of the normals to a curve. Equivalently evolute of a curve is the locus of all its centers of curvature.
Construction
Curve e is evolvent (=involute) of an circle c. Determine a normal line to evolvent e at moving point A. Swich the trace on and determine the envelope of all normals.
Theorem:
A curve is the evolute of any of its evolvents.