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The ancients believed that celestial bodies obeyed different laws than terrestrial objects. By the 17th century, astronomers had shown that Earth is itself a planet, revolving around the Sun like any other. With this insight, Sir Isaac Newton applied the same physical principles that govern everyday motion to explain planetary orbits.
Born in Lincolnshire, England, in 1642, Newton became a Cambridge mathematics professor at 27. His fascination with applying mathematics to physics led him to tackle the mystery of planetary motion, culminating in the publication of the Principia Mathematica in 1687, where he presented the law of universal gravitation.
Before Newton, Kepler’s three empirical laws summarized planetary motion: (1) planets follow elliptical orbits, (2) they sweep equal areas in equal times, and (3) the square of an orbital period is proportional to the cube of its semi‑major axis. These laws described what happened but offered no explanation for why.
Newton insisted that the same forces acting on an apple on Earth also govern the planets. He recognized that in the absence of a force, a body continues in a straight line. To account for the observed elliptical orbits, he inferred an attractive force pulling each planet toward the Sun—a force identical to the one that makes apples fall.
Newton formalized gravity with the equation F = G m₁ m₂ ⁄ r², where G is the gravitational constant, m₁ and m₂ are the masses, and r is the distance between them. When applied to planetary motion, this law reproduces Kepler’s three laws and provides a unified description of both falling bodies and orbital dynamics.
