Early history of Gravity
Since the time of the Greek philosopher Aristotle in the 4th century BC, there have been many attempts to understand and explain gravity. Aristotle believed that there was no effect without a cause, and therefore no motion without a force. He hypothesized that everything tried to move towards their proper place in the crystalline spheres of the heavens, and that physical bodies fell toward the center of the Earth in proportion to their weight.
Another early explanation was that of the Indian astronomer Brahmagupta who, in his Brahmasphuta Siddhanta, responded to critics of the heliocentric system of Aryabhata (476-550) stating that "all heavy things are attracted towards the center of the earth" and that "all heavy things fall down to the earth by a law of nature, for it is the nature of the earth to attract and to keep things, as it is the nature of water to flow, that of fire to burn, and that of wind to set in motion... The earth is the only low thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth."
Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th century and early 17th century. In his famous experiment dropping balls at the Tower of Pisa and later with careful measurements of balls rolling down inclines, Galileo showed that gravitation accelerates all objects at the same rate. This was a major departure from Aristotle's belief that heavier objects are accelerated faster. (Galileo correctly postulated air resistance as the reason that lighter objects appear to fall more slowly.) Galileo's work set the stage for the formulation of Newton's theory of gravity.