When did Newton discover gravity?

Sir Isaac Newton published his theory of gravitation in 1687 in a book known as the "Principia." Newton built upon the discoveries of previous scientists, such as Johannes Kepler and Galileo Galilei, to develop his theory of gravity, also known as the three laws of motion.

A commonly told tale recounts that Newton discovered gravity after an apple fell from a tree and landed on his head. Though this is not strictly true, Newton did first begin to tackle the laws of gravity and planetary attraction after observing the fall of an apple from a tree on his mother's farm as a young man.