THE DISCOVERY OF THE SECOND LAW
The circumstances surrounding the discovery of original laws of science are often very strange and unpredictable. This seems to have been especially true of Kepler's process of arriving at the second law, which says that the velocity of a planet varies in such a way that its radius vector sweeps equal areas in equal times. First of all, although called the second law, it was in actuality discovered first. Kepler proposed it as a postulate in the process of his discovery of the first law; after he had established the first law, he gave a formal proof for the area law.
The discovery of the second law was intimately related to Kepler's attempt to make the sun the dynamic center of the universe and to build up a dynamics of the heavens. Kepler's great ambition was to create a true heavenly physics, as opposed to a heavenly metaphysics or even a heavenly mathematics. To be sure, metaphysics and mathematics had a place in his study of astronomy, but he wanted more: he wanted a system that would give a physical explanation of heavenly phenomena. He wanted to know why the planets moved the way they did and what the laws governing that motion were.
Although the issue of fashioning a real celestial physics would seem completely natural for any astronomer today, it was not a guiding concern in Kepler's time. The mathematical astronomy of Ptolemy had no interest in the investigation of the causes of planetary motion