The book "The Dialogue" was published in
It was written by Galileo Galilei and
summarizes his ideas and views on questions from the
field of astronomy.

The book discusses the two rival astronomical theories - the Ptolemaic, according to which the earth is the center of the world, and the Copernican, which assumes that the sun is immobile and that the planets revolve around it. Galileo examines the two theories in terms of their ability to explain the actual phenomena observed in the heavens, and their correspondence with known terrestrial phenomena. As part of the astronomical discussion, the book also discusses such physical laws as the laws of motion.

The book was written in Italian as a dialogue, i.e., a conversation between three persons, Salivati who represents Galileo's position, Simplicio who represents the Aristotelian philosophers who hold the earth to be immobile, and Sagrado, a Venetian nobleman presented as a layman (i.e., a non-expert) who is willing to learn from the other two speakers. The dialogue weighs the plausibility of the two models of the structure of the universe. It's official aim is the understanding and the joy of the debate itself rather than to judge which theory is true. However, it is obvious that the book accepts the Copernican theory and rejects the Ptolemaic theory. Following the book's publication, Galileo was accused by the Inquisition of belief in the movement of the earth and of teaching that theory, contrary to the explicit prohibition of the Church.

The book is divided into four days (chapters). Each day, the conversation centers on a different issue. On the first day, Galileo speaks against the Aristotelian physics which had been accepted up to his time. According to Aristotle, the supra-lunar world is composed of a special material which is different from earthly material. Ether is a very hard material, but lighter than anything known on earth. The main quality of ether is its circular motion, and thus all the stars, by their nature move in circles. Galileo shows that there is no difference between the material in the heavens and on earth, and thus claims that the stars are not made of ether. The stars and the bodies on earth obey the same laws of nature. Galileo devotes the second part of the dialogue to the law of inertia and shows that there is no difference between movement and rest, and thus that the movement of the earth is possible without any sensation of motion. The third part is devoted to the analysis of astronomical phenomena in order to show that there are formation and destruction in the heavens too, and that not everything on the moon is as eternal and permanent as Aristotle claimed. The fourth day is devoted to the Copernican explanation of the solar system and ebbs and tides on earth. It is interesting to note that in this description, Galileo describes the movement of the planets as circular, although Kepler had already shown twenty years earlier that the orbits of the stars are elliptical.

(The picture is from - GALILEO , RONAN, Colin A. 1974, p. 29)

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