Spherical Trigonometry in the Astronomy of the Medieval Kerala School

Extract

Although the methods of plane trigonometry became the cornerstone of classical Indian mathematical astronomy, the corresponding techniques for exact solution of triangles on the sphere’s surface seem never to have been independently developed within this tradition. Numerous rules nevertheless appear in Sanskrit texts for finding the great-circle arcs representing various astronomical quantities; these were presumably derived not by spherics per se but from plane triangles inside the sphere or from analemmatic projections, and were supplemented by approximate formulas assuming small spherical triangles to be plane.