At http://tallbloke.wordpress.com/2013/03/06/ismael-bullialdus-finder-but-not keeper-of-the-inverse-square-law-of-gravitation/ … a post under the heading of geomagnetism, gravity, solar physics and solar dynamics. Lots of science history. See also HH Ricker at http://tallbloke.files.wordpress.com/2013/03/ricker7.pdf … did Kepler think in terms of a magnetic force which drove planets in their own orbits, towards the Sun and around the Sun? Hans Jelbring replied, the gravitational force proportional to the inverse distance squared is only working at two moments in one orbital circuit. No one can figure out the geometry of the solar system or the motion of the planets and satellites based only on the Newton formula. Kepler provided observational evidence for the constancy … something very important is lacking.