## 13 April 2018

### Epitrochoid

The epitrochoid is a transcendental plane curve, corresponding to a fixed point on a moving circle that rolls without slipping over and externally around another circle, called the director circle. This curve can be drawn through the spirograph.

The parametric equations for a epitrochoid are:

$x=\left( R+r \right) \cos t-d \cos \left( \frac{R+r}{r}t \right)$

$y=\left( R+r \right) \sin t-d \sin \left( \frac{R+r}{r}t \right)$

where $R$ is the radius of the director circle (fixed), $r$ is the radius of the mobile circle, $d$ is the distance from the center point of the mobile circle and $t$ is the parameter of the angle.

For the particular case in which $R=r=d$, we have a cardioid.

In the applet below, click in the Start button to begin the animation that shows the drawing of an epitrochoid. You can use the Stop button to stop the animation at any point. Click in the Reset  button to erase the last curve. You can change the values of R, r and d in the sliders in order to obtain different kinds of epitrochoids. You can also hide the circles and the line segment in order to see only the curve.