Copernicus theorem
If the length of circle B diameter is twice less than the length
of circle A diameter and circle B, remaining internally tangent
to circle A, goes after it without slip, then each point of circle
B moves along the section constituting diameter of circle A.
Theorem formulated by arabic scholar Nasir ad-Din Tusi and
used by Copernicus in his work "De revolutionibus orbium
coelestium (pol. O obrotach sfer niebieskich)".