 11. What trajectories do comets describe? Newton's answer — conic-sections that can at least be approximated by parabolas in the region in which they are observable — gave weight to the question whether the parabolic trajectory works for all comets, and not just the comet of 1680-81 in the case of the first edition, the three others analyzed in the second, and the additional one in the third. The Principia also left open questions about how the gravity of Jupiter and Saturn might affect comet motions, whether any significance should be attached to the residual discrepancies between theory and observation in Newton's results, and which, if any, comets do return in some regular fashion.