Google Classroom
GeoGebraGeoGebra Classroom

How a straight line is an infinite circle

Move the blue points on the sphere to edit the three points defining a circle. See what happens when the points are co-linear. The sphere represents stereographic projection where infinity in all directions are mapped to a 'point at infinity'. https://en.wikipedia.org/wiki/Stereographic_projection See the "Riemann Sphere" file for a follow up activity.