Sir Isaac Newton discovered gravitational force in 1687. The English
mathematician and physicist realized that a force was acting on
falling objects, like apples, because they wouldn't start moving from
rest otherwise. He also realized that the moon would fly off in a
straight line if some force wasn't causing it to fall toward Earth. 
Newton's Law of Universal Gravitation states that any two objects
attract each other with a force equal to the product of their masses
divided by the square of their separation. 
Newton's work, along with that of Galileo, Brahe, and Kepler, proved
that the Earth and other planets orbit the sun, not the other way
around. 
In 1798, British scientist Henry Cavendish conducted an experiment to
accurately determine the value of the gravitational constant, G.