Mathematician Georgios Anastasiou , professor at the University of Memphis, USA, forewords.

The work of the mathematician Theodosius of Tripoli " Spheres" (books 3) is an elegant and rigorously founded set of 60 theorems that comes to fill the gap that exists in Euclid 's " Elements" regarding the sphere and at the same time to provide the required geometric knowledge to the scholar of the celestial sphere. The strict foundation of the work allows the reader to penetrate the propositions of the " Spheres ", to get to know the special methods of the proofs and to ascertain their perfect roughness.