"mah" wrote...
Was toying with my gps recently and plotted a course from my home in the
midwest to London Heathrow. I know it uses great circle routing to
calculate the distance and course. Going from west to east it shows a
course of about 45 degrees. On the return trip it shows an intial
course of 310 degrees.

Now, to my old mind, I would have thought a reciprical course of 225 .
It is something in the spherical formula used for the great circle but
I'm too rusty to work through it.

An old navigators who want to help me out?

A reciprocal initial heading works for a rhumb line (constant heading) course,
but not for a great circle course (except along the Equator or any longitude
line. In all other cases, great circle routes are curved when plotted on a
Mercator map.