Outline of
Selection from Dialogue on the Two Great World Systems by Galileo Galilei

David Banach

I. Summary of Previous Days discussion: (172)

Yesderday we spoke about the natures of the earth and the heavenly bodies.

Today our topic will be whether the earth can be in motion.

Salviati says he does not affirm the Copernican hypothesis but merely presents arguments

Sagredo apologizes for his preseumption

Simplicio says he is intrigued, but remains convinced by the authority of many great writers.

II. Diversion on use of authority: (173)

Medical dissection example: The Aristotelian so trusts Aristotle’s authority that he refuses to believe the testimony of his senses.

III. Beginning of Main Discussion of Earth’s motion. (174 bottom)

A. Earth’s Motion must be imperceptible to us and common to all celestial objects.

B. The Diurnal or daily motion of the heavens from east to west is like this.

C. Initial Objection: (175) Sagredo brings up the expectation of observed parallax (change in observed position) from the seasonal revolution of the earth around the sun. Salviati agrees that this objection is well founded but pots off discussion of the second motion of the earth.

IV. Discussion of Reasons That favor the Mobility of the Earth (and Simplicio’s answers) (176)

1. Immense magnitude of the starry sphere. It makes more sense to have the smaller sphere move rather than the larger.  (Salviati makes the distinction between absolute and relative motion) Since the relative motion in each case would be the same, it is much more reasonable to assume one things moves, than many.
2. Contrary Motions. (178) Having the heavens moves involves attributing to the planets two contrary motions (diurnal and seasonal) Daily the sun and stars move east to west. Over the course of a year the sun (and other planets) move through the zodiac from west to east.
3. Speed of Motion. (179) The greater the size of the sphere, the longer it takes to revolve. But the diurnal motion is fastest of all, being completed in one day for all the spheres, no matter what their size.
4. Different Sizes of Stars’s orbits. (180) The stars move in circles of different sizes depending on their nearness to the poles. So a star near the pole is actually moving much more slowly then one on the equator that has to sweep round a greater circle in the same time.
5. Change in Stars’s orbits.These circles have changed as the stars have moved their positions in the skies over great periods of time. The constellations have moved over great periods the centuries, so the speeds of the stars not only differ from star to star, but are not even constant over time for the same star.
6. Strength of Sphere holding stars. The solidity of the sphere in which the stars are embedded to keep their motions regular also strains belief.
7. Great motion of Heavens would sweep Earth along as well. (181) The motion of the stars which carries all the other spheres along with it, would surely move the earth as well. (Aristotle admitted that weather patterns and other meteorological phenomena (so called because they involved the intersection of the heavens and the earthly spheres) were caused by the motion of the heavens rubbing off on the spheres of fire and air in the earthly realm.

V. Sagredo interrogates Simplicio and summarizes the argument. (181)

A. Sagredo distinguishes between proper motion (motion that belongs to the object itself) and motion by participation. (motion a thing has in virtue of being in a system that is moving, as in the case of a resting person on a moving train.)

B. Sagredo gets Simplicio to admit that the proper motion of the heavens is its seasonal motion and that its daily motion is by participation.

C. Sagredo shows Simplicio that this requires another unobserved sphere that all the other spheres, in whose motion all the other spheres participate for their daily motion. (The Primum Mobile) (See this Diagram) He then shows that this is an unnecessary hypothesis if we can explain the appearances using the earth’s motion instead.

D. Simplicio agrees but says that there are serious problems with attributing motion to the Earth.

VI. Simplicio’s Objections. (183)

A. God’s Power.

1. Simplicio argues that all the difficulties with moving the heavens are of little import if the power that moves them is infinite, as is God.

2. Salviati admits that he would have been wrong to attribute any lack of power to God. He argues that different infinite parts cannot be greater or less than another, so that all the expressions of God’s infinite power large or small are equivalent in worth. He then th main justification of the all the previous arguments “One should not posit more entities to explain a phenomena when fewer will suffice eqally well.” (This is sometimes called Occam’s razor)

3. Salviati questions whether the earth hypothesis explains the appearances equally well.

B. Simplicio’s accoutn of Aristotle’s Objections to the motion of the Earth. (185).

1. Not Perpetual. The Earth’s motion would be violent not natural (which is to the center of the universe), so it could not be perpetual.

2. Parallax. The Earth should have a second seasonal motion, just as do all the other planets. This should result in changes in the observed positions of the stars, which we do not see over the course of the year.

3. Wholes same as Parts. The motion of the whole should be the same as the motion of the parts, which is to the center of the universe.

4. Projectile motion. Objects dropped or shot straight up in the air travel in a straight line up and down instead if being left behind by the motion of the earth as it rotates on its axis.

5. Saves the Appearances. The hypothesis of an immoveable earth accords with the observed appearances of the stars and planets.