Sir Isaac Newton is known for his theory of gravity. His groundbreaking work, particularly encapsulated in his book "Philosophiæ Naturalis Principia Mathematica," published in 1687, introduced the laws of motion and universal gravitation. Newton proposed that every mass attracts every other mass in the universe, which was revolutionary at the time. This gravitational force governs not only celestial bodies, such as planets and moons, but also everyday objects on Earth. Newton's insights laid the foundation for classical physics and significantly advanced our understanding of the natural world. His contributions remain fundamental to various disciplines, including astronomy and engineering.