rotating hemispheres to the viewer

Right now Im working with sets of hemispheres instead of full spheres to save processing time. What Ive got to do is rotate these hemispheres so that from my viewpoint I can`t distinguish a full sphere from a hemisphere, since I´m looking “on top” of that hemisphere. My problem is, that I don´t know how to calculate the correct angles and the order of rotattion axis. I´m pretty sure I´ve got to transform the modelview matrix but I don´t know how.
Thanks for helping & excuse my bad english.

Jan Deiterding