Sir Isaac Newton, an English mathematician and physicist, discovered gravity. Before Newton hypothesized about gravitation in his work "Principia," Galileo Galilee had already laid the foundation for gravitational theory with his work.
Newton's theory was used to predict the existence of another planet beyond Uranus based on odd, unaccounted for deviations in its orbit. John Couch Adams and Urbain Le Verrier calculated that these deviations must be caused by the gravitational force of another planet. However, in the 19th century, slight deviations in Mercury's orbit that could not be accounted for with Newton's theory pointed out its flaws. Einstein's theory of general relativity presented a new concept of gravity which explained many phenomena that Newton's theory could not.