Newton and Kepler's laws: stages of transformation leading toward universal gravitation

Kepler's laws and Newtonian principles There are two widely held opinions concerning the development of Newton's scientific ideas: that he found the law of universal “gravitation” in the 1660s and then refrained from publishing it for twenty years, and that he found this law by “deducing” it from Kepler's “laws” (or, possibly, from only one of Kepler's laws). The analysis presented in this chapter will show that according to any reasonable definition of universal “gravitation”, Newton did not find this law until some time after November 1684, and before 1686, and then published it forthwith. It will be seen that Newton did not (and logically could not) “deduce” the law of universal gravitation from Kepler's laws. In any event, he was not consciously aware of the law of areas in a fruitful context of dynamics until somewhat later than the 1660s, possibly not until the time of a famous exchange of correspondence with Hooke in 1679–1680 (see §3.1). The clarification of the exact role of Kepler's three laws of planetary motion in Newton's thoughts about celestial motions will show Newton's successive steps and transformations leading up to the generalization of a universal force of gravity, and will reveal how the last step entailed a radical transformation of Kepler's laws. This episode provides an example of Newton's creative mind at work, worth far more toward the understanding of whatever logic there may be in scientific discovery than a hundred precepts.