Galileo’s telescopes: Seeing is believing (2010)

Summary

Four hundred years ago, 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 161,1 the moons of Jupiter had become an established fact. These month...