Napier's Right Spherical Triangle
Illustration of a right spherical triangle and the five circular parts placed in the sectors of a circle in the order in which they occur in the triangle. "The ten formulas used in the solution of spherical right triangles can be expressed by means of two rules, known as Napier's rules of circular parts."
George N. Bauer, PH.D. and W.E. Brooke, C.E., M.A. Plane And Spherical Trigonometry (Boston: D.C. Heath & Co., 1917) 138
Courtesy the private collection of Roy Winkelman