Such triangles are composed of arcs of great circles, defined as a circle made by the intersection of a plane through the center of a sphere with its surface. This definition implies a family of great semicircles, going from pole to pole, which are the secondary circles of the coordinate system. Then select the reference secondary circle. The sense, and units, and names are specified. The spherical coordinates on the surface of the earth are longitude and latitude.
Longitude is measured, positive to the east, and latitude positive to the north from the equator.
See Figure A-15 in Appendix 10. To illustrate the use of a spherical triangle on the terrestrial sphere, let us consider a direct flight from Austin to Rome. Copyright 1999 by James N. Douglas. All rights reserved.