In 1639 Viviani, "the last disciple of Galileo," came to live with him at the age of eighteen, and remained on the most intimate terms with him for the rest of the philosopher's life. In 1640 the old man was challenged by one of the Grand Duke's brothers to respond to an argument that the faint light on the "dark" part of the new moon, which Galileo had attributed to earth-shine, was in reality phosphorescence. This Galileo did very effectively with all his old dialectic skill.
Even later he was urged to reply to another argument against the Copernican system, but he had taken to heart the oft-repeated warnings he had received on the subject, and though readily demolishing the point raised against the system, he declared that no good Catholic could doubt the insufficiency of the Copernican doctrine in face of the unanswerable argument of the Divine Omnipotence, but as all the other systems were demonstrably false, it would be necessary to wait for a new one of which both science and theology can approve. It is quite clear that this is only a sarcastic evasion and cannot be understood to mean that he had really changed his mind.
His very last mechanical suggestion was the application of a pendulum to regulate a clock. His son Vincenzio made a drawing of the design from Galileo's dictation, but the plan was interrupted by his mortal illness, and Vincenzio himself also died before completing the clock.
Castelli tried to be with his old master to the end, but had to return to Rome towards the close of 1641. Torricelli, known to fame as the inventor of the barometer, shared with Viviani and Galileo's son the duties of amanuensis, but the sands were now rapidly running out, and the end came on January 8, 1642, when Galileo died after receiving the Pope's blessing.
His enemies were still unappeased, and though they
