1. The problem statement, all variables and given/known data Given the position of the sun (zenith and azimuth angle) and the orientation of a plane (inclination angle and orientation angle). What would be the correct equation to calculate the direction (zenith and azimuth angle) of the specular reflected sun beam as seen from that plane? Example: Sun position: azimuth angle 160° (where azimuth is given clockwise from North (0°)), zenith angle 30°. Plane: Inclination angle 40°, orientation 180° (south). 2. Relevant equations Law of reflection. 3. The attempt at a solution I know that both the incoming and reflected beam form the same angle with respect to the normal to the plane and that the azimuthal directions of the incoming and reflected beam lie both on a plane (thus are shifted by 180°), but did not suceed to figure out the detailed equation to erform the above mentionned calculation for an abritrary sun and plane position. I know the law of reflection (http://en.wikipedia.org/wiki/Specular_reflection) but did not manage to calculate it right. Please help! Thanks alot in advance!!