Newton

Kepler’s laws received a physical explanation only with the publication of English physicist and mathematician Isaac Newton’s Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy, 1687). Here Newton announced his laws of motion, as well as the law of universal gravitation: any two particles in the universe attract one another with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. Newton used these laws to rederive Kepler’s laws, thus making planetary theory a branch of physics for the first time in history. He then applied the laws to explain other phenomena, such as the rise and fall of the tides and the orbits of comets.

The law of inertia (Newton’s first law—a body tends to move at constant speed in a straight line) had been hinted at by Galileo and expressed in a more definite way by French philosopher René Descartes. The third law (if body A exerts a force on body B, then B exerts force on A equal in magnitude but opposite in direction) was well supported by recent work on collisions by Dutch mathematician Christiaan Huygens and others. Newton’s second law (the force impressed on a body is equal to the body’s mass times its acceleration) represented a fresh way of thinking about motion. The idea of an inverse-square law for gravity had been toyed with in England by physicist Robert Hooke, architect Sir Christopher Wren, and astronomer Edmond Halley, but they had been unable to assemble all the necessary concepts—the law of attraction, the concept of motion under an impressed force, and the linking mathematics—into a finished product. Newton’s Principia fundamentally altered the intellectual context for the science of astronomy.

Newton’s law of universal gravitation encountered some resistance, especially on the French-speaking Continent, where it was sometimes regarded as a falling back into a discredited way of thinking. The idea that one body could reach out across empty space and affect another seemed to some to be a throwback to medieval animism. It did not help that Newton could not explain the mechanism by which gravity acted.