Newton's Laws of Motion and Universal Gravitation

In 1687, Isaac Newton published his Principia Mathematica, laying out three laws of motion and the universal law of gravitation — the mathematical framework that explained why planets orbit the Sun, why the Moon stays in place, and why an apple falls to the ground. Using nothing but a quill, paper, and his own invention of calculus, Newton unified terrestrial and celestial mechanics for the first time in history. His equations predicted the return of Halley's Comet, guided the Apollo missions 280 years later, and still govern every satellite in orbit today. It was the foundational act of modern science: the proof that nature obeys discoverable mathematical rules.