34 pages 1 hour read

Galileo Galilei

Discoveries and Opinions of Galileo

Nonfiction | Book | Adult | Published in 1957

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Summary and Study Guide


Discoveries and Opinions of Galileo is a compilation of four writings of Italian astronomer and mathematician Galileo Galilei (1654-1642). Stillman Drake, a specialist in the history and biography of science, translated the Galileo texts from Italian and wrote an introduction and extensive notes for the volume, which was originally published in 1957. The compilation includes the following works: “The Starry Messenger,” “Letters on Sunspots,” “Letter to the Grand Duchess Christina,” and “Excerpts from ‘The Assayer.’”

Galileo played a key role in developing the modern scientific method and in advancing knowledge about the solar system and principles of motion. His writings emphasize the importance of applying reasoned observation to natural phenomena instead of merely relying on authority. Galileo defended his ideas vigorously and at times combatively in his prose writings, which were aimed at educated laypeople rather than scientific experts.

Drake’s commentary situates the writings in the context of Galileo’s career, highlighting the often-heated controversies his discoveries provoked among fellow scholars and representatives of the church, which ended in his being silenced by the Inquisition. Discoveries and Opinions of Galileo is of interest not only to students of the history of science but also to students of early modern literature, as it demonstrates Galileo’s talent as a man of letters. In addition, the writings are relevant to philosophy and theology as well as science.


Part 1, “The Starry Messenger” (1610), is an extended essay in which Galileo describes his findings concerning the moon’s surface and Jupiter’s stars using the newly invented telescope. Galileo’s observations convinced him that the moon has a rough surface and that Jupiter is orbited by four stars in a complex pattern. This work made Galileo famous throughout Italy and Europe and did much to publicize his discoveries and methods, including the use of the telescope.

Part 2, “Letters on Sunspots” (1613), consists of correspondence between Galileo and a German amateur scientist, Mark Welser, about Galileo’s investigation into solar spots—the dark patches that periodically appear on the sun’s surface. Galileo shares with Welser his opinions and observations about the composition, location, and motion of sunspots, while critiquing the opinions of a mutual astronomer friend named Apelles.

Part 3, “Letter to the Grand Duchess Christina” (1615), dates from the period of Galileo’s greatest public controversy, when he was compelled to defend his scientific theories (notably, his advocacy of the Copernican system) in the face of opposition from some theologians. Using quotations from the Bible and early Christian thinkers, Galileo argues that religion and science are autonomous disciplines that complement rather than contradict each other.

Part 4, “Excerpts from ‘The Assayer’” (1623), is Galileo’s response to an attack on him and his ideas by a Jesuit astronomer named Lothario Sarsi. In what Stillman Drake terms “the greatest polemic ever written in physical science” and “Galileo’s scientific manifesto” (227), Galileo explains and defends the use of direct observation and careful reasoning in science, while refuting Sarsi’s claims point by point.

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