mah wrote in message ...
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.

You do not give a lat/long for your home position so a logical answer
is problematical. However a great circle from Heathrow on the angle
of departure you indicate would pass through (roughly) Upper Heyford,
Avon, Kidderminster, Highley and Westbury. However 310 is probably a
rather rough heading for calculations (I know my GPS only gives exact
degrees) so the track may lie considerably to the North or South is
the range is 309 - 311.