As a side note, the airlines and mariners have long been interested in the
solution to this problem. And certain map projections have been designed to
facilitate the solution.
Joel R

 As part of your answer find a formula for the heading angle
 of the geodesic connecting 2 arbitrary fixed points (of known
 latitude and longitude) on the earth's surface as measured by
 an observer at one of those points.

 David Bowman
