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      Avatar photoFrancis Elcaron

      Hi all,

      I’d like to raise a point about Galileo’s argument in this week’s assignment. When I read “Since the secondary light [from the moon] . . . is not received from any star or from the sun, and since in the whole universe there is no other body left but the earth . . .” I thought something had to be wrong, because there was no way Galileo meant that there were only the sun, the stars and the earth in the universe. Taken at face value, as we understand those terms, that proposition is blatantly false and would render the whole argument worthless—but I’m now certain that this is not what Galileo is saying.

      I checked the treatise (*The Starry Messenger,* easily found online as a PDF), which, I think, shows that he is not making a claim about the universe as a whole, but merely vis-à-vis the celestial bodies that had been considered historically as a source of the moon’s secondary light. The word “left” is operative here. In the preceding paragraphs, he argues against claims that the light is inherent in the moon itself, comes directly from the sun by piercing the moon’s solid mass, comes from Venus, or from all the stars together. In the proposition above, therefore, he is saying that he has eliminated all these alternatives, and the only option *left* is the earth (reflecting sunlight).

      A second clarification I want to make is about the word “star.” I’m speaking here as a layman, but as far as I understand, lights in the sky were classically divided roughly into two categories: “fixed stars,” i.e., luminous bodies that appear to keep the same arrangement relative to each other, and “wandering stars,” i.e., planets (the word “planet” comes from a Greek word meaning “wanderer”), which can be observed with the naked eye and move noticeably relative to each other. So Galileo takes planets like Mercury, Venus and Jupiter to be stars, as well as the four satellites of Jupiter that he’s observed for the first time (even naming these last the “Medicean Stars”).

      In light of these points, I take the proposition “. . . in the whole universe there is no other body left but the earth . . .” to be true (though in the assignment I will phrase it differently, in a way I think is appropriate), and I wanted to bring the full context to your attention.

      Enjoy the rest of your Sunday!

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