Toggle "animate" to start the outer (planet) coin rolling/revolving round the inner (sun) coin. Toggle "trace" to leave a trail of the [color=#ff00ff]pink[/color] point, and "curve" to show the path that it travels. The "orbits" slider extends the trace/curve to cover multiple revolutions.[br][br]Notice that in addition to using the slider to set the "ratio" value, you may also type decimals [u]or fractions[/u] into the number field.[br][br]A point on a circle rolling along the outside of a fixed circle traces out an "epicycloid." When ratio = 1, the epicycloid becomes the special case of a "cardioid."[br][br]If the rolling circle is "flipped" to roll inside the fixed circle, a "hypocycloid" is traced.