# Epitrochoid

An **epitrochoid** (/ɛpᵻˈtrɒkɔɪd/ or /ɛpᵻˈtroʊkɔɪd/) is a roulette traced by a point attached to a circle of radius *r* rolling around the outside of a fixed circle of radius *R*, where the point is at a distance *d* from the center of the exterior circle.

The parametric equations for an epitrochoid are

where is a parameter (not the polar angle).

Special cases include the limaçon with *R = r* and the epicycloid with *d = r*.

The classic Spirograph toy traces out epitrochoid and hypotrochoid curves.

The orbits of planets in the once popular geocentric Ptolemaic system are epitrochoids.

The combustion chamber of the Wankel engine is an epitrochoid.

## See also

## References

- J. Dennis Lawrence (1972).
*A catalog of special plane curves*. Dover Publications. pp. 160–164. ISBN 0-486-60288-5.

## External links

- Visual Dictionary of Special Plane Curves on Xah Lee 李杀网
- Interactive simulation of the geocentric graphical representation of planet paths
- O'Connor, John J.; Robertson, Edmund F., "Epitrochoid",
*MacTutor History of Mathematics archive*, University of St Andrews. - Plot Epitrochoid -- GeoFun

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