Giovanni Francesco Sagredo

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Giovanni Francesco Sagredo (1571– 5 March 1620) was a Venetian mathematician and close friend of Galileo, who wrote: Many years ago I was often to be found in the marvelous city of Venice, in discussions with Signore Giovanni Francesco Sagredo, a man of noble extraction and trenchant wit. [1] He was also a friend and correspondent of English scientist William Gilbert.[2]

Sagredo added a scale to Galileo's thermoscope to enable the quantitative measurement of temperature,[3] and produced more convenient portable thermometers.[4] Sagredo also discussed with Galileo the possibility of a telescope using a mirror (a reflecting telescope).[5] Galileo honoured him after his death by making him one of the characters in Dialogue Concerning the Two Chief World Systems discussing the Copernican and the Ptolemaic theories of astronomy.[1]

In 1619, Galileo and Sagredo exchanged portraits.[6] There are two existing portraits, in the Zhytomyr Regional Museum[6][7] and the Ashmolean Museum.[6] They are attributed to the brothers Leandro Bassano and Gerolamo Bassano.[6]

Sagredo died 5 March 1620, leaving his possessions to his brother Zaccaria, who largely disposed of them: Sagredo's collection of letters has not been found.[6] Sagredo's letters to Gilbert were destroyed in the Great Fire of London.[2]


  • Nick Wilding, Galileo's Idol: Gianfrancesco Sagredo and the Politics of Knowledge, University Of Chicago Press, 2014, ISBN 022616697X, 9780226166971


  1. ^ a b Dialogue Concerning the Two Chief World Systems Galileo Galilei, translated by Stillman Drake
  2. ^ a b S. P. Thompson (1903) The Geographical Journal vol 21 no 6, pp 611-618 "William Gilbert and Terrestrial Magnetism"
  3. ^ J. E. Drinkwater (1832) Life of Galileo Galilei page 41
  4. ^ R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 4
  5. ^ Stargazer - By Fred Watson, Inc NetLibrary, Page 109
  6. ^ a b c d e N. Wilding (2006) Galilaeana: Journal of Galilean Studies vol 3, pp.229 – 245 "Galileo’s Idol: Gianfrancesco Sagredo Unveiled"
  7. ^ The Burlington Magazine, Vol. 133, No. 1062 (Sep., 1991), p. 630 (book review)