Definition:Hypocycloid/Generator/Epicycle
This page is about Epicycle of Hypocycloid. For other uses, see epicycle.
Definition
Let a hypocycloid be generated by rolling circle $C_1$ around the inside of another (larger) circle $C_2$.
The circle $C_1$ can be referred to as the epicycle of the hypocycloid.
Also known as
The epicycle of a hypocycloid is also known as its generating circle.
Some sources call it the hypocycle, but (while logically consistent with the meaning of hypocycloid) this is rare.
Some sources fail to name it, and instead refer to the small circle.
It can also be referred to as a rotor.
The term has been taken by $\mathsf{Pr} \infty \mathsf{fWiki}$ from the children's toy Spirograph, which itself was taken from the term rotor as applied to the moving (rotating) part of a rotary machine.