Circle in Circle. Optical illusion

Probably this is the first Cardano's Hypocycloid mechanical model, not animation, that model makers have ever built.
This motion was described first by Nicolaus Copernicus
Consider two circles of radii R and R/2 with the smaller one rolling inside the bigger circle without slipping. Copernicus' Theorem states a surprising result that a point on the circumference of the small circle traces a straight line segment - a diameter of the big circle, to be precise.

The white balls appear to be moving as a circle rolling inside of the lager circle.But what you see is actually not true. This is an optical illusion.The true path of each ball is a straight line.
The mechanical design was made by Girolamo Cardano.
Cardano's work with hypocycloids led him to the Cardan joint or gear mechanism, in which a pair of gears with the smaller being one-half the size of the larger gear is used converting rotational motion to linear motion