Galileo (1564-1642) (fig.1) fabricated his first telescope, with only three magnifications, in the summer of 1609. But already on August 21 of that year, in the bell tower of San Marco (fig.2), in the presence of the Doge and other Venetian notables, he presented an instrument that had eight magnifications, and that won him a lifetime appointment to the Padua Chair of Mathematics at a salary of one thousand florins a year (fig.3) . In November, Galileo had at his disposal a telescope (fig.4) with twenty magnifications, that is, more powerful by far that all the others circulating through Europe at the time, which utilised ordinary lenses made for spectacles, of low quality and with unsuitable focal lengths. The instruments developed by Galileo were highly superior in performance, for example, to the telescope with six magnifications (fig.5) with which the Englishman Thomas Harriot (1560-1621) (fig.6), had conducted observations and made drawings of the lunar surface in July 1609. Thanks to the power of his instrument, Galileo achieved exceptional results in his observations of the moon (fig.7), demonstrating, in fact, that its surface (fig.8) is not perfectly spherical nor immaculate and even managing to calculate the height of the lunar mountains (fig.9). Subsequently, Galileo was to make the exceptional series of astronomical discoveries, described in the Sidereus Nuncius [The Starry Messenger] (Venice, 1610) (fig.10), published in March 1610, and destined to revolutionize forever the traditional view of the cosmos. He was to discover, first of all, the existence of a myriad of new stars (fig.11), showing that the Milky Way is "no other than a mass of innumerable stars scattered in clusters". And again, he was to observe (fig.12, fig.13) the strange appearance of Saturn, whose true cause, the presence of a ring around the planet, was to be found by Christiaan Huygens (1629-1695) nearly half a century later (fig.14). He was the first to observe the phases of Venus (fig.15), which conclusively demonstrated that the planet moved, orbiting around the Sun. But the discovery that brought him immortal fame, in January 1610, was that of the four satellites of Jupiter (fig.16), which Galileo, in homage to the dynasty that ruled Tuscany, named Astri Medicei (fig.17), or Medicean Planets.