First read
Webster 1913's
epicycloid writeup. The cardioid is the
epicycloid for two
circles of
equal radius
a. It is
heart-shaped
in appearance and has
equation
(x2 + y2 - 2ax)2 =
4a2(x2 + y2).
Like all the epicycloids it is interesting from the point of view
of gears. It is a special case of the limaçon of Pascal.