What is a gnomonic projection?
A gnomonic projection maps a sphere onto a plane by projecting all points on the sphere radially, from the sphere’s center, onto a plane that is tangent to the sphere. This projection distorts both angle and area, but it has the useful property that all great circles are projected into straight lines. One can see this by noting that all great circles lie in planes containing the center of the sphere, and that the projection will therefore be the line of intersection between the plane containing the great circle and the plane of the projected map. For more detail, see for instance: http://mathworld.wolfram.com/GnomonicProjection.
