Although his diplomatic attempts to have the Church recognize the heliocentric system and the motion of the Earth had failed, Galileo held firm to his long-time intention of writing a cosmological work. For years he had attentively observed the tides in the sea. He now believed he had found in them physical proof of the Earth’s combined diurnal motion and annual motion. In this he was not entirely wrong, although he had not perfectly understood all the causes of the phenomenon. But the Pope, fearing that the evidence would not be confined to the mathematical sphere alone, refused to accept this subject as the book’s main topic. Accordingly, the Dialogo sopra i due massimi sistemi del mondo [Dialogue on the two chief world systems] covers all of the salient reasons for Galileo’s beliefs: the motion of the Earth, the substance of the Moon, Jupiter’s satellites, sunspots, the relativity of motion, and (half-concealed among the other topics) the tides. It refutes all of the arguments brought against these concepts by the advocates of geocentric and geostatic cosmological systems, by giving better reasons for their greater plausibility, accompanied by unequivocal lessons in methodology. But in spite of its myriad precautions, the Dialogo was prohibited and Galileo condemned to abjuration.
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