Newton's Gravity Reaches Royal Society: Kepler Explained

Edmond Halley walked into the Royal Society on December 10, 1684, carrying a nine-page manuscript from Isaac Newton that would rewrite the laws of the universe. The paper, "De motu corporum in gyrum" ("On the Motion of Bodies in an Orbit"), demonstrated mathematically that an inverse-square law of gravitational attraction explained all three of Johannes Kepler's laws of planetary motion. Newton had solved in a few pages a problem that had confounded the greatest scientific minds in Europe for decades. The chain of events began the previous August, when Halley visited Newton at Cambridge and asked him what curve a planet would follow under an inverse-square attraction to the Sun. Newton reportedly answered immediately: an ellipse. When Halley asked how he knew, Newton said he had calculated it. But he could not find the proof among his papers and promised to reconstruct it. The result was "De motu," which Halley personally brought before the Royal Society. The paper was a revelation, but Newton knew it was incomplete. Halley, recognizing its importance, encouraged Newton to expand the work into a comprehensive treatise. Newton retreated into what may be the most productive period of sustained intellectual effort in human history. For roughly 18 months, he worked with extraordinary intensity, barely eating or sleeping, developing the mathematics of universal gravitation, the three laws of motion, and the foundations of calculus as a physical tool. The result was the "Principia Mathematica," published in 1687 at Halley's personal expense after the Royal Society ran out of funds. The book unified terrestrial and celestial mechanics for the first time, showing that the same force that made an apple fall governed the orbits of planets and the trajectories of comets. Halley's comet itself became one of the Principia's most dramatic confirmations. The nine pages Halley carried into that December meeting contained the seed of modern physics.