      axis toward the +Y axis.

             The earth, moon, and sun are exceptions: planetographic
             longitude is measured positive east for these bodies.

             The default interpretation of longitude by this
             and the other planetographic coordinate conversion
             routines can be overridden; see the discussion in
             Particulars below for details.

             'lon' is output in radians.  The nominal range of 'lon' is
             given by:

              