In 1684 came the famous visit to Newton by the astronomer Edmund Hailey. He had a problem concerning the gravitational attraction between the sun and the planets. Hailey and Hooker had concluded from Johannes Kepler's accounting of planetary motions that the force of attraction mustvary inversely with the square of the distance between a planet and the sun. But they had been unable to prove their idea. "What," Hailey asked Newton, "would be the curve described by the planets on the supposition that gravity diminished as the square of the distance?" Newton answered without hesitation: "An ellipses." How did he know that? "Why," replied Newton, "I havecalculated it. " These four words informed Hailey that Newton had worked out one of the most fundamental laws of the universe — the law of gravity. Hailey wanted to see the calculations at once, but Newton could not find his notes. He promised to write out the theorems and proofs. Under Hailey's insistent urging he completed a manuscript for the Royal Society. Thus was born the Philosophiae Naturalis Principia Mathematica, known ever since simply as the Principia.