rate coefficient giving
             the body's spin sense.

      lon    planetographic longitude of the input point. This is the angle
             between the prime meridian and the meridian containing the input
             point. For bodies having 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 i