Giovanni Francesco Sagredo (19 June 1571 – 5 March 1620) was a Venetian mathematician and close friend of Galileo. He was also a friend and correspondent of English scientist William Gilbert. He is remembered today mainly because he appears as one of the figures in Galileo's controversial work the Dialogue Concerning the Two Chief World Systems (1632).
Family background Sagredo was the fourth of six brothers born to Nicolò Sagredo, son of Bernardo of the S. Sofia branch of the family, and his wife Cecilia, daughter of Paolo Tiepolo. The Sagredo family traced its roots back to the fifth century. His grandfather Bernardo was procurator of S. Marco, while his father, who died in 1615, was appointed administrator of Palmanova in 1600, administrator of Cyprus in 1605, procurator of S. Marco in 1611 and reformer of the University of Padua in 1613. The family lived mainly in Palazzo Sagredo, near the Venetian arsenal and the family church of San Francesco della Vigna, but also owned extensive land holdings in the mainland and elsewhere, including a palace in (it) Marocco and beech forests with iron mines near Cadore. His older brother Bernardo was meant to take on the family's political aspirations, but died in 1603 at the age of 37. The second son, Paolo, disappeared in 1611, while of Stefano, the third son, only a few traces remain. The youngest son Zaccaria had a political career and his son, Nicolò Sagredo became doge. Giovanni Francesco himself did not marry and had no children. He became a member of the Great Council of Venice in 1596 at the age of 25.
Early studies and magnetism Almost everything known of Sagredo is based on the letters that he wrote to Galileo Galilei. About a hundred survive, although those written by Galilei to him are lost. In the 1590s Sagredo studied privately with Galileo, who had come to the University of Padua in 1592. In 1599 Sagredo apologized to Galileo for failure to secure a salary increase for him from the university, although he had used his influential family connections to argue his case. In 1602 Galileo drew up a horoscope for Sagredo, which describes him as "blandum, laetum, hilarem, beneficum, pacificum, sociabilem, pronum ad voluptates, Dei amatorem, laborum impatientem" (kind, happy, merry, beneficient, pacific, sociable pleasure-loving, a lover of God, and impatient of troubles). Sagredo shared an interest in both astronomy and magnetism with Galileo and Paolo Sarpi. By 1600 Sagredo's name was known to Tycho Brahe, who in a letter to Gian Vincenzo Pinelli referred to him as a good contact for the two copies of the star charts that Tycho had offered to the Doge and Senate. Another letter, to William Gilbert describes Sagredo as a "great Magneticall man". One of the hopes that resulted from Gilbert's work was that an all-encompassing mapping of global magnetic declination could be used to solve the problem of calculating longitude. Galilei devised an instrument to measure the magnetic inclination, which Sagredo took with him on his later trip to Syria. Sagredo had a particular contribution to make the advancing the understanding of magnetism - he ran the family mines near Borca di Cadore, from which magnetic iron ore was extracted. Galileo learned how to arm these magnets and the most powerful - known as Rodomonte - was offered for sale to Ferdinando I de' Medici, Grand Duke of Tuscany and to Emperor Rudolf II.
Opposing the Jesuits In April 1606 escalating disputes between Venice and the Vatican led Pope Paul V to decree the Venetian Interdict and Doge Leonardo Donato to expel the Jesuits from the Republic. Shortly afterwards in August 1606 Sagredo was appointed treasurer of the fortress of Palmanova. The Interdict was revoked in April 1607, Sagredo returned to Venice in May and in November he was appointed consul in Syria. From March to July 1608 he concentrated his efforts on an entrapment exercise intended to embarrass the Jesuits. His initial target was the Jesuit scholar Antonio Possevino SJ, rector of the Jesuit college of Ferrara. Possevino had published, under various pseudonyms, pamphlets attacking Venice. Sagredo wrote to the college rector (not knowing that Possevino had by then been replaced by Antonio Barisone). Pretending to be a wealthy widow, Cecilia Contarini, Sagredo wrote to the rector asking for advice on how to get around the Venetian laws which prevented her from leaving the Jesuits a large bequest. The ensuing correspondence came to a head in July when a letter arrived advising Barisone that Cecilia Contarini had died leaving the order 5,000 ducats if he would provide formal documentation confirming that her soul had been received in heaven, before finally making clear to Barisone that he had been fooled. Sagredo then circulated the correspondence widely.
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