Rotation through Radial Translations
The whole image appears to be rotating when the points translate back and forth along lines radiating from the center.[more]
This applies to the fact that a hypocycloid traced by a circle of radius 1 rolling inside a circle of radius 2 is a straight line .[less]
 "How Does a Hypocycloid Straight-Line Mechanism Work?" (Jan 8, 2014) www.mekanizmalar.com/s16.html.