Lénárt sphere-- nice educational tool for exploring spherical geometry; invented by Hungarian István Lénárt as a modern replacement for a spherical blackboard.[1][2] It can be used for visualizing the geometry of points, great and small circles, triangles, polygons, conics, and other objects on a sphere, and comparing spherical geometry to Euclidean geometry as drawn on a flat piece of paper or blackboard.  The included spherical ruler and compass support synthetic straightedge and compass construction on the sphere.