Isaac Newton’s 1687 book, Philosophiae Naturalis Principia Mathematica (known most commonly as the abbreviated Principia). Newton wrote Principia in 18 months, summarizing his work and covering almost every aspect of modern science. Newton introduced gravity as a universal force, explaining that the motion of a planet responds to gravitational forces in inverse proportion to the planet’s mass. Newton was able to explain tides, and the motion of planets, moons, and comets using gravity. He also showed that spinning bodies such as earth are flattened at the poles. The first printing of Principia produced only 500 copies. It was published originally at the expense of his friend, Edmond Halley (1656–1742), because the Royal Society had spent its entire budget on a history of fish.