De Motu Corporum in Gyrum - Halley's Question

Halley's Question

The details of Edmund Halley's visit to Newton in 1684 are known to us only from reminiscences of thirty to forty years later. According to one of these reminiscences, Halley asked Newton, "...what he thought the Curve would be that would be described by the Planets supposing the force of attraction towards the Sun to be reciprocal to the square of their distance from it."

Another version of the question was given by Newton himself, but also about thirty years after the event: he wrote that Halley, asking him "if I knew what figure the Planets described in their Orbs about the Sun was very desirous to have my Demonstration" In light of these differing reports, both produced from old memories, it is hard to know exactly what words Halley used.

It has been sometimes suggested that Newton answered a question different from the one Halley had asked, but any certainty is clearly hard to obtain on this point.