Using the telescope for the first time as a scientific instrument rather than a curious toy, Galileo explored the sky. Before his eyes appeared not only the wonders of nature, but also the physical proof that many of the current astronomical theories, derived from Aristotle, were in reality false. In the Sidereus nuncius (that is, the messenger arriving from the sky to announce the new discoveries) Galileo published the results of his observations of the Moon, the Milky Way, and most notably the four satellites of Jupiter, never before seen. The new discoveries invalidated some of the so-called proofs of the Earth's immobility proclaimed by the advocates of the Aristotelian-Ptolemaic system, thus strengthening Galileo's belief in the Copernican system still further. In the wake of these sensational discoveries, the Grand Duke of Tuscany summoned Galileo to Florence as First Mathematician and Philosopher to the Court.