Ponder...Circle in the Riemann SphereA circle in is either a Euclidean circle in or a Euclidean straight line together with . Definition 5.11 (Circle In ) A circle in is either a Euclidean circle in , or where is a Euclidean straight line in . RelatedRiemann SphereCircles and Möbius TransformationsCross-Ratio Criterion for CirclesStated inDefinition 5.11 (Circle In )§5.3 Circles