Facing the Facts: Few Events in History Have Proved as Momentous as Galileo's Discovery of the Moons of Jupiter. David Wootton Explains Why

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Four hundred years ago this summer Galileo Galilei (1564-1642) was in a state of anxiety. In January he had discovered four moons orbiting Jupiter. In March he had published this and other remarkable discoveries made with his improved telescope in Sidereus Nuncius ('The Starry Messenger'). But by the summer he was becoming profoundly alarmed. He had offered philosophers and mathematicians in Venice, Padua, Florence, Pisa and Bologna the chance to look through his telescope and confirm his discoveries. Some, including Cremonini, the highest paid academic in Italy and a good friend of Galileo's, had simply refused. Others had looked, but had said they could not see what on earth he was talking about. Only Kepler in Germany had come out in his support, but he had yet to get hold of a decent telescope so had not seen the moons for himself. He was prepared to believe Galileo because he--and hardly anyone else--shared Galileo's Copernicanism.

Galileo had to wait patiently that summer as Jupiter had disappeared from the night sky. He was also in something of a quandary; if he gave good telescopes to other astronomers, then they might make new discoveries with his equipment (Galileo's most important telescopic discovery, the phases of Venus, which more or less destroyed Ptolemaic astronomy, was to come within a few months, in December 1610). Worse, if he gave them telescopes and they were unable to make sense of what they saw, his reputation might be permanently damaged.


What he wanted was for them to make their own telescopes. In the autumn this began to happen--Thomas Harriot in England and Nicolas-Claude Fabri de Peiresc in France saw the moons for themselves, but no one told Galileo. Kepler got hold of a Galilean telescope and confirmed Galileo's discoveries. Above all, in Rome the Jesuit mathematicians finally announced in December that they had confirmed all of Galileo's discoveries. By the spring of 1611 the moons of Jupiter had become an established fact.

These months, from March to December 1610, saw the birth of modern science. Before 1610 no one had ever thought that the task of scientists was to make discoveries. As Galileo complained in a letter to Kepler in August, philosophers assumed that sound knowledge came from comparing texts, not developing new instruments (Cremonini went on to write a long book on the heavens, promptly banned by the Catholic Church, in which the telescope and Galileo's discoveries are never mentioned). It is safe to say that prior to 1610 not a single significant scientific argument had turned on a question of fact. Now Galileo began to be compared by his contemporaries with Vespucci, Columbus and Magellan, the discoverers of new lands.


During the last 400 years science has transformed our world as discovery has succeeded discovery. …