  Newton apparently discovered the systematic relationship between conic-section trajectories and inverse-square central forces at the time, but did not communicate it to anyone, and for reasons that remain unclear did not follow up this discovery until Halley, during a visit in the summer of 1684, put the same question to him. His immediate answer was, an ellipse; and when he was unable to produce the paper on which he had made this determination, he agreed to forward an account to Halley in London. Newton fulfilled this commitment in November by sending Halley a nine-folio-page manuscript, “De Motu Corporum in Gyrum” (“On the Motion of Bodies in Orbit”), which was entered into the Register of the Royal Society in early December 1684. The body of this tract consists of ten deduced propositions — three theorems and seven problems — all of which, along with their corollaries, recur in important propositions in the Principia.