 Newton, by calculating from Kepler's laws, and supposing the orbits of the planets to be circles with the sun at the centre, had already proved that the force of the Sun acting upon the different planets must vary as the inverse square of the distances of the planets from the Sun.He was therefore led to inquire whether if the Earth's attraction extended to the Moon, the force at that distance would be of the exact magnitude necessary to retain the Moon in its orbit.He found that the Moon by its motion in its orbit was deflected from the tangent in every minute through a space of 13 feet (3.96 m).