It is stated that "The culminating moment of the Scientific Revolution was the discovery made by Isaac Newton of the law of universal gravitation." With a simple law, Newton hinted at the most important physical phenomena of the observable universe.
The law of universal gravitation was born in 1685 as the culmination of a series of studies and works initiated much earlier. In 1551, the Spaniard Domingo de Sotofue first established that a body in free fall suffers a constant acceleration. The first written reference we have of the idea of universal attraction is 1666, in the book Micrographia, by Robert Hooke. In 1679 Robert Hooke introduced Newton to the problem of analyzing a curved trajectory. When Hooke became secretary of the Royal Society he wanted to establish a philosophical correspondence with Newton. In his first letter he raised two questions that would deeply interest Newton. Until then scientists and philosophers like Descartes and Huygens analyzed the curvilinear movement with the centrifugal force. Hooke, however, proposed "to compose the celestial movements of the planets from a rectilinear movement along the tangent and an attractive movement, towards the central body." It suggests that the centripetal force towards the Sun varies in inverse ratio to the square of the distances. Newton replies that he had never heard of this hypothesis.
In 1684 Newton informed his friend Edmund Halley that he had solved the problem of force inversely proportional to the square of the distance. Newton wrote these calculations in the De Motu treatise and developed them extensively in the book Philosophiae naturalis principia mathematica. Although many astronomers did not use Kepler's laws, Newton sensed their great importance and magnified them by demonstrating them from his law of universal gravitation.
However, universal gravitation is much more than a force directed towards the Sun. It is also an effect of the planets on the Sun and on all the objects of the Universe. Newton intuited easily from his third law of dynamics that if an object attracts a second object, this second also attracts the first with the same force. Newton realized that the movement of the celestial bodies could not be regular. He affirmed: "the planets neither move exactly in ellipses, nor turn twice according to the same orbit". For Newton, fervent religious, the stability of the orbits of the planets implied continuous readjustments on their trajectories imposed by the divine power.
