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Astronomical Images : The Copernican system

Galileo Galilei

Astronomical Images

<p style='text-align: justify;'>In Day 3 of the <i>Dialogo</i>, Galileo's interlocutors Salviati, Sagredo, and Simplicio debate the merits of the Copernican system in light of evidence drawn from astronomical observations. Of particular interest to the interlocutors is the question of Copernicus's proposed annual motion of the Earth. To address this question, Galileo employed several diagrams, one of which is illustrated here. This diagram is preceded by a long discussion, with no images, in which Galileo's interlocutor Salviati argues that astronomical evidence necessitates that the Earth not be the centre of the Universe and that the planets must revolve around the Sun. The diagram is introduced after Simplicio remarks that he is 'not yet convinced of this arrangement at all.' He suggests to Salviati that 'Perhaps I should understand it better by carrying out some <i>disegno</i>, which might make it easier to discuss.' Galileo, through Salviati, then relies on text and image in tandem to present the Copernican system as a logical necessity given the available astronomical evidence. The accompanying text is constructed to read as a record of a dialogue between the three interlocutors, a dialogue in which the image is drawn as the interlocutors speak. Galileo's mouthpiece in the dialogue, Salviati, begins by proposing that Simplicio himself draw the diagram using paper and a compass. After having Simplicio mark the Sun on his paper, Salviati mentions specific astronomical evidence that Simplicio must take into account as he adds in the remaining planets. Salviati's comments are suggestive of a master and pupil relationship. Salviati, the teacher, leads Simplicio to the truth by letting him discover it himself. As presented by Galileo, this process alludes to Plato's theory of knowledge as recollection, set out in his <i>Meno</i>, among other works. According to this theory, gaining knowledge amounts to recollecting knowledge the soul previously had. As a result, Socrates' method of pedagogy, a dialectic exchange between teacher and pupil in which the teacher prompts the pupil to come to knowledge of a subject by him- or herself, is similar to that Salviati adopts with Simplicio in the <i>Dialogo</i>.</p>

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