Essay by ahpatrick93College, UndergraduateB, September 2014

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Galileo's three letters to Mark Welser on Sunspots

Consult Stillman Drake, "Introduction: Second Part," XV-XVI (pp; 81-85) for an overview of the debate between Galileo and Christopher Scheiner, a Jesuit who writes under the pseudonym Apelles latens post tabulam (Appelles for short).

Key questions and issues for our consideration:

1. What is at issue between Appelles and Galileo? Material scatted throughout Galileo's correspondence with Welser sheds light on this

question. Galileo provides a convenient summary of their disagreement toward the end of his first letter, p 102.

Galileo provides a careful description of the phenomena that both he and Appelles have observed in his first letter, pp. 98-99. He describes the sunspots again in this second letter on p. 106 and proceeds with his analysis on pp. 107-109.

For a summary of his own conclusions and the arguments supporting them, see for example pp. 112-115 and p. 143.

2. How is one to observe the sunspots (as you may well have been wondering)? See the instructions on pp. 115-116.

3. On more than one occasion in his letters, Galileo takes the Aristotelians (the Peripatetics), but not Aristotle, to task. On what grounds does he excuse Aristotle but condemn the Aristotelians?

For answers see pp. 118-119, 127, 142.

4. A number of passages worthy of reflection and quotation are sprinkled throughout Galileo's letters. Here are references to some of my favorites:

p. 92 (theme: names and attributes)

p. 97 (what "philosophical astronomers seek to investigate")

pp. 123-124 (knowing properties contrasted with knowing essences)

pp. 136 (Nature deaf to our entreaties)

pp. 137 (remarks on the "inhabitants" of other planets)

pp. 139 (names and essential properties; true stars contrasted with spots)

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