Having the start/end points of a great-circle, compute their bearings.

Here's how it works:

Because earth is (almost) a sphere, the shortest distance from, say, Spain to Japan is not a straight line, but a great-circle line, that looks like this. So a plane would start its flight at a 29° bearing in Spain but land at 150° in Japan. That's how the geometry works.

Enter either:

• decimal latitudes/longitudes with minus sign for South and West
• degrees minutes seconds in a format like E 32 14 9 (32°14'9" East longitude).