This article deals with the methodological procedures employed by Johannes Kepler, particularly those used in the process of elaboration of the two first laws of planetary movements. Its aim is to show that the astronomical practice of Kepler is linked with the proposal of (physical and mathematical) hypothesis and with valuing precision in observational data, with the goal of obtaining, by means of rigorous procedures, the (mathematically expressed) regularities of planetary motions. It was only afterwards that Kepler looked for an (...) explanation of the regularities (laws) discovered, formulating the hypothesis of the magnetic action exerted by the Sun, which is located at the physical center of the system. In the new astronomy, he modifies the methodological status of astronomical hypotheses in the beginning of 17th century (present in Ptolemy, Copernicus and Brahe), promoting the passage from a mathematical descriptive astronomy to a physical-mathematical explicative astronomy. (shrink)