While other scientists before him had observed and theorized about gravity, Newton's groundbreaking work in his book "Philosophiæ Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy) in 1687 laid the foundation for our understanding of gravity. He:

* Formulated the law of universal gravitation: This law states that every particle in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them.

* Explained the motion of planets and other celestial bodies: Newton's law of gravitation could explain the elliptical orbits of planets around the sun and the tides caused by the moon.

While Einstein later refined our understanding of gravity with his theory of general relativity, Newton's work remains foundational and is still used for many practical applications today.