Giovanni Francesco Sagredo (1571– 5 March 1620) was a Venetian

Venice

Venice is a city in northern Italy which is renowned for the beauty of its setting, its architecture and its artworks. It is the capital of the Veneto region...

 mathematician

Mathematician

A mathematician is a person whose primary area of study is the field of mathematics. Mathematicians are concerned with quantity, structure, space, and change....

 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

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, and produced more convenient portable thermometer

Thermometer

Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

s. Sagredo also discussed with Galileo the possibility of a telescope using a mirror. Galileo honoured him after his death by making him one of the characters in Dialogue Concerning the Two Chief World Systems

Dialogue Concerning the Two Chief World Systems

The Dialogue Concerning the Two Chief World Systems was a 1632 Italian language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was translated to Latin as Systema cosmicum in 1635 by Matthias Bernegger...

 discussing the Copernican

Nicolaus Copernicus

Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

 and the Ptolemaic

Ptolemy

Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 theories of astronomy.

In 1619, Galileo and Sagredo exchanged portraits. There are two existing portraits, in the Zhytomyr

Zhytomyr

Zhytomyr is a city in the North of the western half of Ukraine. It is the administrative center of the Zhytomyr Oblast , as well as the administrative center of the surrounding Zhytomyr Raion...

 Regional Museum and the Ashmolean Museum

Ashmolean Museum

The Ashmolean Museum on Beaumont Street, Oxford, England, is the world's first university museum...

. They are attributed to the brothers Leandro Bassano

Leandro Bassano

Leandro Bassano , also called Leandro dal Ponte, was an Italian artist from Bassano del Grappa, the younger brother of Francesco Bassano the Younger and third son of Jacopo Bassano, who took their name from their town of Bassano del Grappa...

 and Gerolamo Bassano

Girolamo da Ponte

Girolamo da Ponte also known as Gerolamo Bassano was an Italian painter of the Renaissance period.Born at Bassano del Grappa 3 June 1566, he was the youngest son of painter Jacopo da Bassano. He also copied his father's pictures, and like his brother Giambattista, must take his share in the...

.

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

Great Fire of London

The Great Fire of London was a major conflagration that swept through the central parts of the English city of London, from Sunday, 2 September to Wednesday, 5 September 1666. The fire gutted the medieval City of London inside the old Roman City Wall...

.