 5. What are the true motions of the planets, and which, if any, of the schemes for calculating planet locations is to be preferred, Kepler's or one of the alternatives to it? Newton's answer was not simple: “If the sun were at rest and the remaining planets did not act upon one another, their orbits would be elliptical, having the sun in their common focus, and they would describe areas proportional to the times;” and the aphelia and nodes would be stationary. The Keplerian system, amended in the manner of Horrocks to infer mean distances from the periods, is therefore the preferred approximation to the true motions. The main loose-end in this answer was whether the actual motions do deviate from the Keplerian ideal, and if so, whether all the deviations could be attributed to specific forces, gravitational or otherwise. A further loose-end, addressed in part in Book 3, was whether the non-Keplerian motion of the Moon can be shown not to be a counterexample to Newton's argument in the case of the planets.