Gian Domenico Cassini in Bologna and his contributions to the assessment of the planetary system

G. D. Cassini was hired as a professor of astronomy in Bologna in 1649, at the age of only 24, and left for Paris in 1669, when he was 44. We will outline in this Paper his major contributions to astronomy during those year according to the judgement of the scholars : the demonstration of the physical inequality of the Suns motion using the meridian line he had built in the church of S. Petronio and the discovery of Mars and Jupiter rotation. The first result unequivocally dismissed Aristotles celestial mechanics based on uniform circular motions, pointing to the unity of the terrestrial and celestial worlds; the second contributed to renew the debate on whether the dynamical structure of the planetary system was Ptolemaic or Copernican. In this respect, some recently discovered lessons, which Cassini gave in Bologna in the year 1666, appear enlightening. Excerpts are reported and commented upon.