Hypotrochoid

A hypotrochoid is a roulette traced by a point attached to a circle of radius r rolling around the inside of a fixed circle of radius R, where the point is a distance d from the center of the interior circle.

The parametric equations for a hypotrochoid are:

Where ${\displaystyle \theta }$ is the angle formed by the horizontal and the center of the rolling circle (these are not polar equations because ${\displaystyle \theta }$ is not the polar angle).

Special cases include the hypocycloid with d = r and the ellipse with R = 2r.

The classic Spirograph toy traces out hypotrochoid and epitrochoid curves.

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