Kepler made measurements of the motion of each planet. He showed that for all planets the cube of the radius of orbit was proportional to the square of the time period. (r3 / T2 is constant).Newton assumed an inverse square law for gravity and went on to prove that the gravitational force between any two objects is always attractive, is proportional to the product of their masses and inversely proportional to the square of their distance apart. The constant of proportionality is G the universal constant of gravitation.