The Geometry of the Sphere by John C. Polking
Publisher: Rice University 2000
We are interested here in the geometry of an ordinary sphere. In plane geometry we study points, lines, triangles, polygons, etc. On the sphere we have points, but there are no straight lines. Therefore it is natural to use great circles as replacements for lines. Then we can talk about triangles and polygons and other geometrical objects.
Mathematics Geometry & Topology