                                 Sun 
 
                  surface normal vector 
                            ._                 _. 
                            |\                 /|  Sun vector 
                              \    phase      / 
                               \   .    .    / 
                               .            . 
                                 \   ___   / 
                            .     \/     \/ 
                                  _\ s.i./ 
                           .    /   \ 