---
title: "Dialogue Concerning the Two Chief World Systems"
chunk: 2/6
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"
---

Direct physical argument for the Earth's motion, by means of an explanation of tides.
As an account of the causation of tides or a proof of the Earth's motion, it is a failure. The fundamental argument is internally inconsistent and actually leads to the conclusion that tides do not exist. But, Galileo was fond of the argument and devoted the "Fourth Day" of the discussion to it. The degree of its failure is—like nearly anything having to do with Galileo—a matter of controversy.  On the one hand, the whole thing has recently been described in print as "cockamamie." On the other hand, Einstein used a rather different description:

It was Galileo's longing for a mechanical proof of the motion of the earth which misled him into formulating a wrong theory of the tides. The fascinating arguments in the last conversation would hardly have been accepted as proof by Galileo, had his temperament not got the better of him.  [Emphasis added]

=== Omissions ===

The Dialogue does not treat the Tychonic system, which was becoming the preferred system of many astronomers at the time of publication and which was ultimately proven incorrect.  The Tychonic system is a motionless Earth system but not a Ptolemaic system;  it is a hybrid system of the Copernican and Ptolemaic models.  Mercury and Venus orbit the Sun (as in the Copernican system) in small circles, while the Sun in turn orbits a stationary Earth;  Mars, Jupiter, and Saturn orbit the Sun in much larger circles, which means they also orbit the Earth.  The Tychonian system is mathematically equivalent to the Copernican system, except that the Copernican system predicts a stellar parallax, while the Tychonian system predicts none.  Stellar parallax was not measurable until the 19th century, and therefore there was at the time no valid disproof of the Tychonic system on empirical grounds, nor any decisive observational evidence for the Copernican system.
Galileo never took Tycho's system seriously, as can be seen in his correspondence, regarding it as an inadequate and physically unsatisfactory compromise. A reason for the absence of Tycho's system (in spite of many references to Tycho and his work in the book) may be sought in Galileo's theory of the tides, which provided the original title and organizing principle of the Dialogue. While the Copernican and Tychonic systems are equivalent geometrically, they are quite different dynamically. Galileo's tidal theory entailed the actual, physical movement of the Earth; that is, if true, it would have provided the kind of proof that Foucault's pendulum apparently provided two centuries later. Without reference to Galileo's tidal theory, there would be no difference between the Copernican and Tychonic systems.
Galileo fails to discuss the possibility of non-circular orbits, although Johannes Kepler had sent him a copy of his 1609 book, Astronomia nova, in which he proposes elliptical orbits—correctly calculating that of Mars. Prince Federico Cesi's letter to Galileo of 1612 treated the two laws of planetary motion presented in the book as common knowledge; Kepler's third law was published in 1619. Four and a half decades after Galileo's death, Isaac Newton published his laws of motion and gravity, from which a heliocentric system with planets in approximately elliptical orbits is deducible.

== Summary ==
"Preface: To the Discerning Reader" refers to the ban on the "Pythagorean opinion that the earth moves" and says that the author "takes the Copernican side with a pure mathematical hypothesis". He introduces the friends Sagredo and Salviati with whom he had had discussions as well as the peripatetic philosopher Simplicio.