Newton Publishes the Principia and Explains Why the Planets Orbit
One law of gravitation accounts for falling apples, orbiting moons, and comets alike
Quick facts
- Published
- 1687 CE
- Full title
- Philosophiae Naturalis Principia Mathematica
- Key persuader
- Edmond Halley, 1684
- Core law
- Universal gravitation, inverse-square force
What happened
Isaac Newton published Philosophiae Naturalis Principia Mathematica in 1687, after Edmond Halley persuaded him in 1684 to write up and formalize work Newton had developed piecemeal since the 1660s. The Principia's central law states that all matter attracts all other matter with a force proportional to the product of their masses and inversely proportional to the square of the distance between them, universal gravitation. Newton used this single law to show that the planets were attracted toward the Sun by a force varying as the inverse square of the distance, deriving Kepler's previously empirical laws of planetary motion as mathematical consequences of gravity rather than as independent rules, and he extended the same force to explain the orbits of comets, the tides, and the motion of the Moon as perturbed by the Sun's gravity.
Why it matters
Newton's Principia supplied the physical mechanism that Copernicus's and Kepler's mathematical descriptions of the solar system had lacked, explaining in a single unified law why planets follow elliptical orbits at all rather than simply describing that they do. It is widely regarded as the greatest scientific book ever written, and it turned astronomy from a descriptive, geometric discipline into a predictive branch of physics.
How we know
The Principia survives in its original 1687 printing and subsequent editions Newton himself revised, and its mathematical derivations can be checked directly against Kepler's earlier laws, which the Principia shows follow logically from the inverse-square gravitational force.
Sources
- MacTutor History of Mathematics, University of St Andrews. Isaac Newton · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- MacTutor History of Mathematics, University of St Andrews. Johannes Kepler · Reputable sourcemathshistory.st-andrews.ac.uk · The domain "mathshistory.st-andrews.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- The Scientific Revolution → · Newton's Principia is generally treated as the capstone of the Scientific Revolution; see that timeline for how it built on Galileo's and Kepler's earlier work.