外圆滚线

[ noun ]
a line generated by a point on a circle rolling around another circle
<noun.shape>

Epicycloid \Ep`i*cy"cloid\, n. [Epicycle + -oid: cf. F.
['e]picyclo["i]de.] (Geom.)
A curve traced by a point in the circumference of a circle
which rolls on the convex side of a fixed circle.

Note: Any point rigidly connected with the rolling circle,
but not in its circumference, traces a curve called an
epitrochoid. The curve traced by a point in the
circumference of the rolling circle when it rolls on
the concave side of a fixed circle is called a
hypocycloid; the curve traced by a point rigidly
connected with the rolling circle in this case, but not
its circumference, is called a hypotrochoid. All the
curves mentioned above belong to the class class called
roulettes or trochoids. See {Trochoid}.