<P> To make this into an equal - sided formula or equation, there needed to be a multiplying factor or constant that would give the correct force of gravity no matter the value of the masses or distance between them . This gravitational constant was first measured in 1797 by Henry Cavendish . </P> <P> In 1907 Albert Einstein, in what was described by him as "the happiest thought of my life", realized that an observer who is falling from the roof of a house experiences no gravitational field . In other words, gravitation was exactly equivalent to acceleration . Between 1911 and 1915 this idea, initially stated as the equivalence principle, was formally developed into Einstein's theory of general relativity . </P> <P> In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse - square law of universal gravitation . In his own words, "I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly ." </P> <P> Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted by the actions of the other planets . Calculations by John Couch Adams and Urbain Le Verrier both predicted the general position of the planet, and Le Verrier's calculations are what led Johann Gottfried Galle to the discovery of Neptune . </P>

Who came up with the theory of gravity