How to find the great circle angle given two points on the surface of the sphere?

Hi
I am not familiar with spherical geometry. I am working with elliptical polarization that involves using poincare sphere that present the latitude and longitude angle in spherical geometry. I need to find the great circle angle if given two points that each specified by their longitude angle

and latitude angle

.

ie. If I am given the

and

of

and

, how can I find the great circle angle between

and

?

I really don"t want to learn the details of spherical geometry, just want to learn the way of finding the angle as this is only a small part of antenna design.

Thanks

Alan