public function onEnterFrame(event:Event):void {
    // the book seems to make a mistake here
     // ball.x = centerX + Math.sin(angle) * radiusX;
     // ball.y = centerY + Math.cos(angle) * radiusY;
     ball.x = centerX + Math.cos(angle) * radiusX;
     ball.y = centerY + Math.sin(angle) * radiusY;
     angle += speed;
    }
   }
  }