  This analysis of the “deduction” of universal gravity does not answer two further complaints lodged against it. First, in concluding that the centripetal force acting on the Moon is inverse-square, Newton grants that the precession of the lunar orbit implies an exponent of -2 and 4/243 for the force rather than exactly -2, but then claims that the small fraction can be accounted for by the perturbing action of the Sun's gravity. But the magnitude for the action of the Sun that he gives in Proposition  3[49] is twice the value he later in Book 3 indicates is the correct value. This lacuna was not resolved by Alexis-Claude Clairaut until two decades after Newton died. Second, when Newton invokes the third law of motion in the corollaries to Proposition 5, he is tacitly assuming that, for example, Jupiter and the Sun are, in effect, directly interacting. In other words, he is ignoring the alternative favored by Huygens that some unseen medium is effecting the centripetal force on Jupiter, a medium that can in principle absorb the linear momentum which Newton is assuming is being transferred to the Sun. Huygens may well have perceived this lacuna, to which Cotes explicitly called Newton's attention while he was preparing the second  edition.[50]