The Routledge Guidebook to Galileo's Dialogue (The Routledge Guides to the Great Books) by Maurice A. Finocchiaro

Author:Maurice A. Finocchiaro [Finocchiaro, Maurice A.]
Language: eng
Format: azw3
ISBN: 9781136010965
Publisher: Taylor and Francis
Published: 2013-07-30T16:00:00+00:00

IIIC1. HELIOCENTRISM OF PLANETARY REVOLUTIONS (DML 370–80)

The heliocentrism of planetary revolutions is a handy label for the thesis that the planets Mercury, Venus, Mars, Jupiter, and Saturn revolve in orbits whose center is the sun (rather than the earth). This particular thesis does not regard the earth as one of the planets, and so does not assert that the earth too revolves around the sun. The thesis is referring only to the five bodies which both sides of the controversy agreed were planets, moving westward in orbits of different sizes, and taking different periods of time to complete them. The earth's annual heliocentric motion is a separate issue, which Galileo does also address in this section, but he does so only after he has established that the center of the orbits of these five planets is the sun.

In fact, this passage contains an argument with two main steps. The first is a subargument supporting the heliocentrism of planetary revolutions based on evidence that is partly old and partly new. The old evidence is easily obtained with naked-eye observation, and was known since ancient times, and was completely uncontroversial in Galileo's time. The new evidence relies on the telescope and stems from Galileo's own discoveries; although it had been controversial at first, by the time he wrote the Dialogue it, too, was relatively uncontroversial. Indeed, by this time, the heliocentrism of planetary motion was itself a thesis that was widely accepted and relatively uncontroversial. In particular, it provided the central tenet in Tycho's system, according to which the planets revolve around the sun, although the sun with the whole planetary system moves daily and annually around a motionless and central earth. Thus, what gives interest to this part of Galileo's argument is not the conclusion it reaches, but rather the clarity, incisiveness, and elegance of his presentation of the evidence.

The second step of the main argument is a subargument supporting the conclusion that the earth too revolves annually around the sun, based on the heliocentrism of planetary revolutions—the thesis previously established. This is the more controversial part of the argument; as we shall see, it is weak, but ingenious and plausible.

To prove the heliocentrism of planetary revolutions, Galileo reviews the evidence regarding each planet in turn. In presenting this evidence, he uses several semi-technical terms, which it is proper for us to explain at the outset. These terms are labels used to refer to the relative position of two heavenly bodies on the celestial sphere, in terms of the angular distance between them as measured by a terrestrial observer. Two bodies are said to be in opposition (to each other) when they appear on opposite sides of the celestial sphere, namely 180 degrees apart. They are said to be in conjunction when they appear next or near each other on the celestial sphere, namely at an angular distance close to zero. They are said to be in quadrature when they appear about 90 degrees apart on the celestial sphere.

The argument begins with Venus, regarding which the evidence is most detailed.

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