Kepler’s laws of planetary motion, published in the first decades of the seventeenth century, gave astronomers a better picture of how the planets moved, but didn’t explain what caused that movement. One popular theory was that of philosopher Rene Descartes, who postulated that the universe was made up of regions of invisible matter, called vortices, one of which contained the solar system. The Sun would be at the center of the vortex, and the invisible matter would carry the planets in a whirlpool fashion around the Sun.

English scientists, including Robert Hooke and other members of the Royal Society (a scientific academy founded in 1660), based their studies of planetary motion on William Gilbert’s sixteenth-century work on magnetism. In the 1660’s and 1670’s, Hooke studied the attractive powers of the Earth, Moon, and Sun, concluding that all bodies in the solar system possess an attractive power like the gravity which holds objects to the surface of the Earth. He further concluded that the planets attract other planets, and that these attractive forces diminish as the distance between the objects increase. Over the next decade, English scientists established that the attraction between celestial bodies must vary inversely with the distance between them squared (the “inverse square law”), but were unable to prove this mathematically.

Isaac Newton, a mathematics professor at Cambridge University, had tackled the problems of gravitation and planetary orbits on his own throughout the 1660’s and 1670’s. He found the available mathematical tools, arithmetic and geometry, to be inadequate for his use, so he created a new mathematics – calculus – to calculate the effects of gravitation on the motion of celestial bodies. In 1684, when Edmund Halley approached Newton for confirmation that planetary orbits conformed to the inverse square law, Newton was persuaded to share his mathematical proofs with the Royal Society. His drafts culminated in the publication of the Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), also known as the Principia, in 1687. The Principia explains Newton’s calculus and principles of dynamics, and discusses his three laws of motion and the law of gravity. Tying together the work of Kepler and Galileo, the Principia proved that the planetary orbits and falling objects on Earth were governed by the same universal force, gravity, and that the effect of gravitational forces on the planets produced their elliptical orbits. Since Copernicus first proposed that the Earth and the other planets revolved around the Sun, Newton was the first person to comprehensively explain how and why the solar system operated.