The Lénárt sphere is a clear plastic sphere with hemispherical transparent overlays, designed by István Lénárt as an aid for teaching various concepts in non-Euclidean geometry, in particular, comparing the properties of geometrical constructs in spherical geometry with those they exhibit in plane geometry. The kit includes an 8″-diameter plastic sphere with a toroid stand (at center in the photograph above), several hemispherical transparent overlays (one shown at left), four wet-erase felt-tip pens for drawing on them (two shown), a spherical straight edge/ruler (at right, marked in degrees), and a spherical compass (bottom right, also marked in degrees). Not shown is the center locator for the compass. (There should also be a spherical protractor, but, unfortunately, it is missing.)

As the description above indicates, you can draw lines and polygons on the surface of the sphere (on the transparent overlays), discuss their properties, and compare them to their analogues drawn on a flat plane, or to their projections onto a plane. The spherical compass allows you to draw circles of any diameter up to that of a great circle (i.e., one having the diameter of the sphere itself). You can also model the celestial sphere with this device by drawing astronomical reference objects at appropriate points on the sphere, in case you can illustrate their relationships more clearly with the Lénárt sphere than with demonstration 92.06 -- Celestial globe.