Define the spherical triangle with sides A, B, and C (measured by subtending angles) and opposing angles alpha, beta, and gamma. Given the three angles, calculate any of the three sides (i.e. subtending angles). In this case, return the side alpha across from angle A. Note that in a spherical triangle, the sum of the three angles must be > 180 degrees.
This is often used in aerospace engineering where the spherical triangle applies to plotting geographic shapes on the surface of the Earth.