This is inexcusably tardy (especially if already mentioned and I missed it), but Gene's http://people.csail.mit.edu/bkph/other/Stanford_AI_WP_79_Salamin.pdf has a wonderful derivation of rotation matrices prior to diving into quaternions. --rwg pi/2 + 2/pi?! quad> This was so obvious in retrospect, although I'm battling the corner cases (and the sign of T) now. Thanks! Robert On Saturday, June 7, 2014, Dan Asimov <dasimov@earthlink.net> wrote: In almost all cases, V := (A-A')x(B-B') will be a vector in the direction of the axis of rotation. Knowing V makes it easy to project say A and A' onto the perpendicular plane to C to determine the angle T. The remaining cases should be easy to exclude or deal with. --Dan On Jun 7, 2014, at 1:32 AM, Robert Smith <quad@symbo1ics.com <javascript:;>> wrote: Let A and B be unit vectors in R^3. Suppose they are rotated about some vector V by an angle T, resulting in A' and B' respectively. What are V and T? I set up a quadratic system using quaternions and got a result that was 3 million terms large. Am I missing something?