---
title: "Dialogue Concerning the Two Chief World Systems"
chunk: 6/6
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 four ===
They are in Sagredo's house in Venice, where tides are an important issue, and Salviati wants to show the effect of the Earth's movement on the tides. He first points out the three periods of the tides: daily (diurnal), generally with intervals of 6 hours of rising and six more of falling; monthly, seemingly from the Moon, which increases or decreases these tides; and annual, leading to different sizes at the equinoxes. He considers first the daily motion. Three varieties are observed: in some places the waters rise and fall without any forward motion; in others they move towards the east and back to the west without rising or falling; in still others there is a combination of both—this happens in Venice where the waters rise on entering and fall on leaving. In the Straits of Messina there are very swift currents between Scylla and Charybdis. In the open Mediterranean the alteration of height is small but the currents are noticeable.
Simplicio counters with the peripatetic explanations, which are based on the depths of the sea, and the dominion of the Moon over the water, though this does not explain the risings when the Moon is below the horizon. But he admits it could be a miracle. When the water in Venice rises, where does it come from? There is little rise in Corfu or Dubrovnik. From the ocean through the Straits of Gibraltar? It's much too far away and the currents are too slow. So could the movement of the container cause the disturbance? Consider the  barges that bring water into Venice. When they hit an obstacle, the water rushes forward; when they speed up it will go to the back. For all this disturbance there is no need for new water and the level in the middle stays largely constant though the water there rushes backwards and forwards.
Consider a point on the Earth under the joint action of the annual and diurnal movements. At one time these are added together and 12 hours later they act against each other, so there is an alternate speeding up and slowing down. So the ocean basins are affected in the same way as the barge particularly in an east-west direction. The length of the barge makes a difference to the speed of oscillations, just as the length of a plumb bob changes its speed. The depth of water also makes a difference to the size of vibrations. The primary effect only explains tides once a day; one must look elsewhere for the six-hour change, to the oscillation periods of the water. In some places, such as the Hellespont and the Aegean the periods are briefer and variable. But a north-south sea like the Red Sea has very little tide whereas the Messina Strait carries the pent up effect of two basins.
Simplicio objects that if this accounts for the water, should it not even more be seen in the winds? Salviati suggests that the containing basins are not so effective and the air does not sustain its motion. Nevertheless, these forces are seen by the steady winds from east to west in the oceans in the tropical zone. It seems that the Moon also is taking part in the production of the daily effects, but that is repugnant to his mind. The motions of the Moon have caused great difficulty to astronomers. It's impossible to make a full account of these things given the irregular nature of the sea basins.

== See also ==
"Discourse on the Tides", 1616 Galileo essay

== Further reading ==
Kelly, Henry Ansgar (2026). "Galileo's Three Repudiations of Copernicanism – Two Coerced and One Volunteered". The Historical Journal.

== Notes ==

== Bibliography ==
Drake, Stillman (1970). Galileo Studies. Ann Arbor: The University of Michigan Press. ISBN 0-472-08283-3.
Linton, Christopher M. (2004). From Eudoxus to Einstein – A History of Mathematical Astronomy. Cambridge: Cambridge University Press. ISBN 978-0-521-82750-8.
Sharratt, Michael (1994). Galileo: Decisive Innovator. Cambridge: Cambridge University Press. ISBN 0-521-56671-1.

== External links ==

 Media related to Dialogo sopra i due massimi sistemi del mondo at Wikimedia Commons
Italian text with figures (in Italian)
Thomas Salusbury's 1661 English translation of the Dialogue. Online copy of full text.
Dialogo dei massimi sistemi. Fiorenza, Per Gio: Batista Landini, 1632. From the Rare Book and Special Collections Division at the Library of Congress
Audio book version by Brian Keating