Giovanni Francesco Sagredo
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.  He was also a friend and correspondent of English scientist William Gilbert.
Sagredo added a scale to Galileo's thermoscope to enable the quantitative measurement of temperature, and produced more convenient portable thermometers. Sagredo also discussed with Galileo the possibility of a telescope using a mirror (a reflecting telescope). 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.
In 1619, Galileo and Sagredo exchanged portraits. There are two existing portraits, in the Zhytomyr Regional Museum and the Ashmolean Museum. They are attributed to the brothers Leandro Bassano and Gerolamo Bassano.
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. Sagredo's letters to Gilbert were destroyed in the Great Fire of London.
- Nick Wilding, Galileo's Idol: Gianfrancesco Sagredo and the Politics of Knowledge, University Of Chicago Press, 2014, ISBN 022616697X, 9780226166971
- Dialogue Concerning the Two Chief World Systems Galileo Galilei, translated by Stillman Drake
- S. P. Thompson (1903) The Geographical Journal vol 21 no 6, pp 611-618 "William Gilbert and Terrestrial Magnetism"
- J. E. Drinkwater (1832) Life of Galileo Galilei page 41
- R. P. Benedict (1984) Fundamentals of Temperature, Pressure, and Flow Measurements, 3rd ed, ISBN 0-471-89383-8 page 4
- Stargazer - By Fred Watson, Inc NetLibrary, Page 109
- N. Wilding (2006) Galilaeana: Journal of Galilean Studies vol 3, pp.229 – 245 "Galileo’s Idol: Gianfrancesco Sagredo Unveiled"
- The Burlington Magazine, Vol. 133, No. 1062 (Sep., 1991), p. 630 (book review)
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