an epicycloid in which the rolling circle equals the fixed circle
cardioid详细解释
car.di.oid n.（名词）A heart-shaped plane curve, the locus of a fixed point on a circle that rolls on the circumference of another circle with the same radius.心脏线：一种心形的平滑曲线，圆周上一固定点绕另一具有相同半径的圆的圆周转动的轨迹
