Newton's Principia Published
Isaac Newton published Principia Mathematica, laying the foundations of classical mechanics
July 05, 1687
The Book That Explained the Universe
In 1687, Isaac Newton published "Philosophiae Naturalis Principia Mathematica" — known simply as the Principia. It is widely considered the most important scientific book ever written. In its pages, Newton laid out three laws of motion that describe how objects move and interact, and introduced the law of universal gravitation: every object in the universe attracts every other object with a force proportional to their masses and inversely proportional to the square of the distance between them. This single law explained why objects fall, why the Moon orbits Earth, and why planets orbit the Sun.
What Newton Unified
Before Newton, the motions of the planets and the behavior of objects on Earth were thought to be governed by completely different rules. Galileo had studied motion on Earth; Kepler had described the paths of the planets with precise mathematics. Newton's genius was to show that one set of laws — his laws of gravity and motion — governed both. He proved that Kepler's planetary orbits were a natural consequence of his gravitational law. For the first time, the cosmos was governed by mathematics that any person could, in principle, learn and use.
A Legacy That Still Guides Spacecraft
Newton's laws remained the foundation of physics for more than 200 years. Even after Einstein's general relativity replaced them at extreme scales, Newton's equations remain accurate enough to guide spacecraft across the solar system. When NASA engineers calculate the trajectory of a Mars rover or a lunar lander, they rely on Newton's Principia as their foundation. The book was published with the financial support of astronomer Edmond Halley — who also used Newton's gravity to predict the return of the comet that now bears his name. The Principia remains the template for what a scientific theory should look like.