---
title: "Dialogue Concerning the Two Chief World Systems"
chunk: 3/6
source: "https://en.wikipedia.org/wiki/Dialogue_Concerning_the_Two_Chief_World_Systems"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:36:10.787237+00:00"
instance: "kb-cron"
---

=== Day one ===
Salviati starts with Aristotle's proof of the completeness and perfection of the world (i.e. the universe) because of its three dimensions. Simplicio points out that three was favoured by the Pythagoreans whereas Salviati cannot understand why three legs are better than two or four. He suggests that the numbers were "trifles which later spread among the vulgar" and that their definitions, such as those of straight lines and right angles, were more useful in establishing the dimensions. Simplicio's response was that Aristotle thought that in physical matters mathematical demonstration was not always needed.
Salviati attacks Aristotle's definition of the heavens as incorruptible and unchanging whilst only the lunar-bound zone shows change. He points to the changes seen in the skies: the new stars of 1572 and 1604 and sunspots, seen through the new telescope. There is a discussion about Aristotle's use of a priori arguments. Salviati suggests that Aristotle uses Aristotle’s personal experience to choose an appropriate argument to prove just as others do and that Aristotle would change his mind in the present circumstances.
Simplicio argues that sunspots could simply be small opaque objects passing in front of the Sun, but Salviati points out that some appear or disappear randomly and those at the edge are flattened, unlike separate bodies. Therefore, "it is better Aristotelian philosophy to say 'Heaven is alterable because my senses tell me' than 'Heaven is unalterable because Aristotle was so persuaded by reasoning.'" He adds "we possess a much better basis for reasoning about celestial things than Aristotle did...Now we, thanks to the telescope, have brought the heavens thirty or forty times closer to us than they were to Aristotle, so that we can discern many things in them that he could not see; among other things these sunspots, which were absolutely invisible to him." Experiments with a mirror are used to show that the Moon's surface must be opaque and not a perfect crystal sphere as Simplicio believes. He refuses to accept that mountains on the Moon cause shadows, or that reflected light from the Earth is responsible for the faint outline in a crescent moon.
Sagredo holds that he considers the Earth noble because of the changes in it whereas Simplicio says that change in the Moon or stars would be useless because they do not benefit man. Salviati points out that days on the Moon are a month long and despite the varied terrain that the telescope has disclosed, it would not sustain life. Humans acquire mathematical truths slowly and hesitantly, whereas God knows the full infinity of them intuitively. And when one looks into the marvelous things men have understood and contrived, then clearly the human mind is one of the most excellent of God's works.

=== Day two ===
The second day starts by repeating that Aristotle would be changing his opinions if he saw what they were seeing. "It is the followers of Aristotle who have crowned him with authority, not he who has usurped or appropriated it to himself." There is one supreme motion—that by which the Sun, Moon, planets and fixed stars appear to be moved from east to west in the space of 24 hours. This may as logically belong to the Earth alone as to the rest of the universe. Aristotle and Ptolemy, who understood this, do not argue against any other motion than this diurnal one. Motion is relative: the position of the sacks of grain on a ship can be identical at the end of the voyage despite the movement of the ship. Why should we believe that nature moves all these extremely large bodies with inconceivable velocities rather than simply moving the moderately sized Earth? If the Earth is removed from the picture, what happens to all the movement?
The movement of the skies from east to west is the opposite of all the other motions of the heavenly bodies which are from west to east; making the Earth rotate brings it into line with all the others. Although Aristotle argues that circular motions are not contraries, they could still lead to collisions. The great orbits of the planets take longer than the shorter: Saturn and Jupiter take many years, Mars two, whereas the Moon takes only a month. Jupiter's moons take even less. This is not changed if the Earth rotates every day, but if the Earth is stationary then we suddenly find that the sphere of the fixed stars rotates in 24 hours. Given the distances, that would more reasonably be thousands of years. In addition some of these stars have to travel faster than others: if the Pole Star was precisely at the axis, then it would be entirely stationary whereas those of the equator have unimaginable speed. The solidity of this supposed sphere is incomprehensible. Make the Earth the primum mobile and the need for this extra sphere disappears.