    prograde (aka direct) rotation, the direction of 
              increasing longitude is positive west:  from the +X 
              axis of the rectangular coordinate system toward the 
              -Y axis.  For bodies having retrograde rotation, the 
              direction of increasing longitude is positive east: 
              from the +X axis toward the +Y axis. 
 
              The earth, moon, and sun are exceptions: 
              planetographic longitude is measured positive east for 
        