---
title: "Dialogue Concerning the Two Chief World Systems"
chunk: 4/6
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"
---

They consider three main objections to the motion of the Earth: that a falling body would be left behind by the Earth and thus fall far to the west of its point of release; that a cannonball fired to the west would similarly fly much further than one fired to the east; and that a cannonball fired vertically would also land far to the west. Salviati shows that these do not take account of the impetus of the cannon. He also points out that attempting to prove that the Earth does not move by using vertical fall commits the logical fault of paralogism (assuming what is to be proved), because if the Earth is moving then it is only in appearance that it is falling vertically; in fact it is falling at a slant, as happens with a cannonball rising through the cannon (illustrated).
In rebutting a work which claims that a ball falling from the Moon would take six days to arrive, the odd-number rule is introduced: a body falling 1 unit in an interval would fall 3 units in the next interval, 5 units in the subsequent one, etc. This gives rise to the rule by which the distance fallen is according to the square of the time. Using this he calculates the time is really little more than 3 hours. He also points out that density of the material does not make much difference: a lead ball might only accelerate twice as fast as one of cork. In fact, a ball falling from such a height would not fall behind but ahead of the vertical because the rotational motion would be in ever-decreasing circles. What makes the Earth move is similar to whatever moves Mars or Jupiter and is the same as that which pulls the stone to Earth. Calling it gravity does not explain what it is.

=== Day three ===
Salviati starts by dismissing the arguments of a book against the novas he has been reading overnight. Unlike comets, these were stationary and their lack of parallax easily checked and thus could not have been in the sublunary sphere. Simplicio now gives the greatest argument against the annual motion of the Earth that if it moves then it can no longer be the center of the zodiac, the world. Aristotle gives proofs that the universe is finite bounded and spherical. Salviati points out that these disappear if he denies him the assumption that it is movable, but allows the assumption initially in order not to multiply disputes.

Salviati points out that if anything is the center, it must be the Sun not the Earth, because all the planets are closer or further away from the Earth at different times, Venus and Mars up to eight times. He encourages Simplicio to make a plan of the planets, starting with Venus and Mercury which are easily seen to rotate about the Sun. Mars must also go about the Sun (as well as the Earth) since it is never seen horned, unlike Venus now seen through the telescope; similarly with Jupiter and Saturn. Earth, which is between Mars with a period of two years and Venus with nine months, has a period of a year which may more elegantly be attributed to motion than a state of rest.
Sagredo brings up two other common objections. If the Earth rotated, the mountains would soon be in a position that one would have to descend them rather than ascend.  Secondly, the motion would be so rapid that someone at the bottom of a well would have only a brief instance to glimpse a star as it traversed. Simplicio can see that the first is no different from travelling over the globe, as any who have circumnavigated but though he realizes the second is the same as if the heavens were rotating, he still does not understand it. Salviati says the first is no different from those who deny the antipodes. For the second, he encourages Simplicio to decide what fraction of the sky can be seen from down the well.
Salviati brings up another problem, which is that Mars and Venus are not as variable as the theory would suggest. He explains that the size of a star to the human eye is affected by the brightness and the sizes are not real. This is resolved by use of the telescope which also shows the crescent shape of Venus. A further objection to the movement of the Earth, the unique existence of the Moon, has been resolved by the discovery of the moons of Jupiter, which would appear like Earth's Moon to any Jovian.