Isaac Newton compared the movement of the moon to the movements of objects on earth. Believing that gravitational forces were responsible for each. Newton was able to conclude that gravity depended upon distance. This comparison led him to a conclusion that the force of gravitational attraction between the Earth and other objects is inversely proportional to the distance separating the earth’s center from
the object’s center. But distance is not the only variable affecting the magnitude of a gravitational force. Consider Newton’s famous equation
(Fnet = m • a)                                                

Newton knew that gravity acted on the apple making it fall to the ground was caused by gravity.  Since the force acting to cause the apple’s downward motion also causes the earth’s upward acceleration (Newton’s third law) that force must also depend on the mass of the earth.