Q:  Roughly how large (in degrees) is the offset in longitude
            between the sub-stellar point and the "hot spot"?


        Visual inspection of Figure 11 from Stevenson et al.
        suggests (to my eyes) that the "hot spot" is offset from the
        sub-stellar location = longitude 0 by about 30 degrees.

        The proper analysis by the authors places this offset 
        at somewhere between 10 and 20 degrees.