GALILEO GALILEI 39 



and therefore partakers in the movement, would be unconscious of it, 

 as if it did not occur, since we can only take into consideration earthly 

 things. Therefore it is necessary that this movement should seem to 

 belong to all the other bodies and visible objects in common which, 

 separated from the earth, Have no share in its movement. The correct 

 method of determining whether movement is to be attributed to the 

 earth, and what movement, is that one should inquire and observe 

 whether an apparent movement can be ascribed to the bodies outside 

 of the earth, which belongs to all of them in the same degree. So a 

 movement which, for example, can be supposed of the moon, and not 

 of Venus or Jupiter or other stars, cannot be peculiar to the earth. 

 Now there is such a general movement governing all other objects, 

 namely that which the sun, moon, planets, fixed stars, in a word the 

 whole universe with the single exception of the earth, seems to follow 

 from east to west within the space of twenty-four hours. This, at 

 least at first glance, may be just as well attributed to the earth alone, 

 as to the rest of the entire universe except the earth. 



Sagr. : I understand clearly that your suggestion is correct. An 

 objection, however, forces itself upon me that I cannot solve. That 

 is, since Copernicus ascribes to the earth a further movement aside 

 from the daily one, according to the above mentioned principle this 

 should be apparently un-noticeable on the earth, but should be visible 

 in the rest of the universe. I come then to the conclusion that either 

 he plainly erred when he ascribed to the earth a movement to which no 

 counterpart is apparent in the firmament, or else such a movement ex- 

 ists, and then Ptolemaus is guilty of a second error in that he did not 

 refute with arguments this movement as well as that daily rotation. 



Salv. : Your objection is very just. If we take up this other 

 movement, you shall see how much superior in intelligence was 

 Copernicus to Ptolemaus, in that he saw what this one did not, namely 

 how wonderfully this second motion is reflected in the rest of the 

 heavenly bodies. For the present, however, we will leave this aside 

 and return to our first consideration. Proceeding from the most 

 general suppositions, I will present the arguments which seem to 

 favor the motion of the earth, in order then to hear the opposing 

 arguments of Signore Simplicio. First, then, when we consider the 

 immense circumference of the stellar sphere in comparison with the 



