Newton publishes the Principia

Isaac Newton's Principia set out his three laws of motion and the law of universal gravitation. It became the base of classical physics.

Context

Edmond Halley visited Newton in August 1684 and asked about the dynamics of orbits. Newton promised a demonstration, and Halley received the short tract De Motu about three months later. Over the following two and a half years, that tract grew into the Principia, published in 1687.

The three laws are usually summarised this way: an object stays in uniform motion unless an external force acts on it, force equals mass times acceleration, and every action has an equal and opposite reaction. Newton stated the second law in terms of change of motion, so the modern formula is a later restatement.

The law of universal gravitation holds that every particle of matter attracts every other with a force that depends on their masses and falls off with the square of the distance between them. Newton used it to explain elliptical orbits of celestial bodies.

HISTORY says the book made Newton famous in intellectual circles, earned wide acclaim as a landmark of modern science, and became a foundation of the European Enlightenment. Britannica adds that the work led from the three laws to universal gravitation, treating motion on Earth and in the heavens under one set of rules. Halley's 1684 visit is the usual starting point of the story, with De Motu as the seed of the book.