The animation shows how an hypocycloid is formed. A hypocycloid is "the curve produced by a fixed point P on the circumference of a circle with radius b rolling without sliding around the inside of a large circle of radius a, with a>b." (definition by: http://mathworld.wolfram.com/Hypocycloid.html)
This dynamic interactive sheet has some problems: the angle B'O ̂B after having reached 359.99°, instead of keep growing, comes back to zero. For this it is impossible to close hypocycloids that need more than one rotation of the circumference β inside the circumference α cannot be drawn.
It is also impossible to draw hypocycloids with the generating circumferences with a negative radius.
