Definition:Epitrochoid

Definition
Let a circle $C_1$ of radius $b$ roll around the outside of another circle $C_2$ of radius $a$.

Consider a point $P$ on the line of a radius of $C_1$ at a distance $d$ from the center of $C_1$.

The curve traced out by $P$ is called an epitrochoid.

Epicycloid
The special case where $d = b$ is usually treated separately:

Also see

 * Equation of Epitrochoid