The Sun comprises 99.9% of the solar system mass so it is expected that Sun terrestrial planet interactions can influence the motion as well as the rotation of the terrestrial planets.Gravity affects the planet orbital motions while the changing magnetic fields of the Sun can influence the planet rotations. Planets that manifest a magnetic field dominate any weaker magnetic fields from the Sun, but the rotation of terrestrial planets without a magnetic field interacts with the changing Sun’s field dependent on the electrical conductivity of the core region. It is determined that the average planet density becomes a useful quantity to describe the magnetic state of a terrestrial planet. An average density of 5350 ± 50 kg/m3 is hypothesized to separate planets that develop magnetospheres from those that do not. Planets with higher average densities,Mercury and Earth, developed magnetospheres. While those with lower average densities, Venus and Mars never developed magnetospheres. Terrestrial planets with magnetospheres are the ones to also exhibit plate tectonics.The small size of Mercury led to Mercury only exhibiting a frozen in magnetiza-tion of potentially magnetic regions. The lack of magnetospheres as well as lack of plate tectonicsprevented the continual transfer of core heat to the surface that limited the surface vulcanism to an initial phase. For Venus, it meant that the surface regions would only sporadically convulse. In this picture,the apparent anomalous axial rotation of Venus is a natural consequence of the rotation of the Sun. For Mars with relatively low surface temperatures,it meant that there was little heat exchange through the crust that would allow the lower crust to retain large amounts of water. For Mars to have initially had flowing liquid water required that the atmosphere at that time contained high concentrations of infrared absorbing gases at least as compared to the present level of infrared absorbing gases on the Earth. The terrestrial planets have iron based cores because iron has the highest binding energy per nucleon that can be made in the steady state lives of massive stars no matter how massive. This suggests that many of the conclusions reached here may also be applicable to exoplanets.