# How did Kepler arrive at his laws?

How did Kepler arrive at his laws? If one already knows the distances to the planets (and the eccentricity of the orbits, etc.), it is understandable that one might proceed to establish Kepler's second and third laws. However, how did Kepler do this without knowing the distances? His third law seems a true revolution since it establishes the distances between the Sun and the planets (in astronomical units at least); it just seems such a huge and daring step ahead. What he was basing this on?

• I believe that people knew the distances, in astronomical units, of the known planets from the Sun before Kepler. This is how he built his "platonic solid" model of the solar system. You only need distances in AU, and these can be gleaned by parallax and triangulation. Some idea of absolutes was also known around this time in terms of diameters of the Earth (again I think this must have been by triangulation) - Ole Roemer calculated the speed of light to be 7600 times the Earth's speed around the Sun, whence Huygens deduced $c$ was 16.7 Earth diameters each second. Feb 17 '14 at 10:32
• @WetSavannaAnimalakaRodVance thank you, I did not know about this! I found physics.stackexchange.com/a/36011/37125 , so just the notion of heliocentrism gave the means to measure the planet distances from the sun in AU. This is so beautyful. Feb 17 '14 at 10:51
• Yes, I agree, Ron's answer is an extremely insightful take on it. However, it would be good to get a summary of exactly how the measurement history went from a real historian/astronomer, because the other question didn't really get such an answer. Feb 17 '14 at 12:19
• Also see this. Sep 23 '15 at 20:02
• @raxacoricofallapatorius this is interesting, thank you Sep 24 '15 at 9:04