Just as students have difficulty with Newton’s third law as discussed in the article by Lehavi and Galili,1 it is of interest to realize that Newton’s understanding of the significance of the mutual interaction of bodies in their motion did not occur until very late leading up to his writing of the Principia. Cohen2 published an article on this, which also appeared as an appendix in a revised and updated version of his book.
Newton wrote an essay entitled “De motu” in which he described a calculation of the orbit of a planet in our solar system.2 He ran into the problem that his calculation did not fit the actual data sufficiently. He realized that assuming a stationary Sun was a potential source of error. He recalculated, but this time he allowed the Sun to move in response to the pull of the planet. The revised...
