Halley coaxed the Principia out of Newton — and paid for it

On 10 December 1684, the astronomer Edmond Halley told the Royal Society that he had lately seen Isaac Newton at Cambridge, who had shown him “a curious treatise, De Motu”, and had promised to send it for the Society’s register. That brief report is the first public trace of the work that became the Principia.

Halley later described to Newton how it began. In January 1684, having reasoned that the force holding planets in orbit weakens with the square of distance, he met Christopher Wren and Robert Hooke in London. Hooke claimed he could derive all celestial motions from that principle. Wren, unconvinced, offered a book worth 40 shillings to whichever of them brought a convincing proof within two months. Hooke, Halley noted, never made good on his promise to show it.

In August Halley visited Newton with the same question: what orbit does a body trace under an inverse-square force? Newton had solved it about five years earlier and told no one. In November the answer arrived, the nine-page tract De Motu Corporum in Gyrum, showing that the orbit is an ellipse, or at higher speeds a parabola or hyperbola.

From tract to book

• February 1685: Newton’s propositions on motion are entered in the Society’s register, with 10 December 1684 noted in the margin.
• 28 April 1686: Book 1 of the Principia manuscript, dedicated to the Society, is presented at a meeting.
• 2 June 1686: with the Society short of funds, still recovering from the cost of printing Willughby’s Historia Piscium, its council orders the book printed, with Halley to oversee it “at his own charge”.
• 1687: the Principia is published.

Then Hooke raised a priority claim on the inverse-square law, and an angry Newton recast the final book. Halley kept the project alive anyway, and the book founded classical mechanics.