The Works of Galileo

A New Pope and a New Dialogue

Dialogue, the text

Galileo was already famous across Europe and didn’t have to write in Latin. He wrote this in Italian, and as a dialogue. (Figure 1) It was a best-seller. To entertain it did. The book was a literary masterpiece, cast in the form of a Platonic dialogue between three characters (Figure 2) :

  • Salviati, named for a close friend of Galileo’s, defends the Copernican system with scientific brilliance and legendary wit. Master of both quantitative argument and experimental evidence, Salviati always seems ten steps ahead of anyone else. Never at a loss for words, Salviati clearly directs the flow of conversation.
  • Sagredo, named after another friend of Galileo’s, represents you or me on our best days. As an inquisitive and bright student, he is not intimidated by Salviati, but comes up with insightful questions. Sagredo is the man in the middle; he desires neither to discard traditional authorities nor to embrace novelties unless there is compelling evidence.
  • And then there is Simplicio, named after an ancient Greek commentator on Aristotle, who attempts to defend the Earth-centered system. Simplicio frequently admits that he doesn’t understand an argument, and his requests for additional explanations provide comic relief along the way.

The three characters engage in conversations over four days:

  1. The first day critiques the celestial/terrestrial dichotomy of Aristotelian cosmology;
  2. the second day explains the relativity of motion in the heavens;
  3. the third day explores the annual motion of the Earth, and an argument from sunspots;
  4. and the fourth day is devoted to Galileo’s argument from the tides.

Astronomical arguments for Copernicanism advanced in the Dialogo include:

  • Phases of Venus prove that it orbits the Sun.
  • Variation in motion of the sunspots, consistent with annual motion of Earth plus rotation of the Sun (evidence by analogy for rotation of the Earth as well).
  • Similarity between surfaces of Earth and Moon.
  • Satellites of Jupiter prove more than one center of revolution in the universe.
  • Discovery of new stars, sufficiently distant to explain the absence of parallax, without requiring them to be illuminated by the Sun.

The Dialogo made these arguments persuasively for the rest of the century. (See Noel Swerdlow, in Cambridge Companion to Galileo, 1998).

Figure 1 Figure 1 - Return to Text

Dialogue (1632), title page.

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Figure 2 Figure 2 - Return to Text

Dialogue (1632), first page.

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Exhibit credit: Kerry Magruder, with the assistance of , Marilyn B. Ogilvie, Duane H. D. Roller.

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