.. or grit your teeth, and have a look at
Ed Williams's Aviation Formulary V1.31 
Some great circle formulae:
Distance between points
The great circle distance d between two points with coordinates {lat1,lon1} and {lat2,lon2} is given by:
d=acos(sin(lat1)*sin(lat2)+cos(lat1)*cos(lat2)*cos(lon1-lon2))
A mathematically equivalent formula, which is less subject to rounding error for short distances is:
d=2*asin(sqrt((sin((lat1-lat2)/2))^2 + cos(lat1)*cos(lat2)*(sin((lon1-lon2)/2))^2))
[ 17 October 2001: Message edited by: Checkboard ]