---
title: "Dialogue Concerning the Two Chief World Systems"
chunk: 5/6
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"
---

Copernicus has succeeded in reducing some of the uneven motions of Ptolemy who had to deal with motions that sometimes go fast, sometimes slow, and sometimes backwards, by means of vast epicycles. Mars, above the Sun's sphere, often falls far below it, then soars above it. These anomalies are cured by the annual movement of the Earth. This is explained by a diagram in which the varying motion of Jupiter is shown using the Earth's orbit.
Simplicio produces another booklet in which theological arguments are mixed with astronomic, but Salviati refuses to address the issues from Scripture. So he produces the argument that the fixed stars must be at an inconceivable distance with the smallest larger than the whole orbit of the Earth. Salviati explains that this all comes from a misrepresentation of what Copernicus said, resulting in a huge over-calculation of the size of a sixth magnitude star. But many other famous astronomers over-estimated the size of stars by ignoring the brightness factor. Not even Tycho, with his accurate instruments, set himself to measure the size of any star except the Sun and Moon. But Salviati (Galileo) was able to make a reasonable estimate simply by hanging a cord to obscure the star and measuring the distance from eye to cord.
But still many cannot believe that the fixed stars can individually be as big or bigger than the Sun. To what end are these? Salviati maintains that "it is brash for our feebleness to attempt to judge the reasons for God's actions, and to call everything in the universe vain and superfluous which does not serve us".
Has Tycho or any of his disciples tried to investigate in any way phenomena that might affirm or deny the movement of the Earth? Do any of them know how much variation is needed in the fixed stars? Simplicio objects to conceding that the distance of the fixed stars is too great for it to be detectable. Salviati points out how difficult it is even to detect the varying distances of Saturn. Many of the positions of the fixed stars are not known accurately and far better instruments than Tycho's are needed: say using a sight with a fixed position 60 miles away.
Sagredo then asks Salviati to explain how the Copernican system explains the seasons and inequalities of night and day. This he does with the aid of a diagram showing the position of the Earth in the four seasons. He points out how much simpler it is than the Ptolemaic system. But Simplicio thinks Aristotle was wise to avoid too much geometry. He prefers Aristotle's axiom to avoid more than one simple motion at a time.