GALILEO GALILEI 43 



words. I know that in fact it is simpler and more natural to ac- 

 complish everything with one motion than to call in two. If you do 

 not wish to call them opposite, then call them reverse. Moreover, I 

 mention this introduction of a double movement not as something im- 

 possible, and in no way propose to deduce from it a strong proof for 

 the motion of the earth, but merely a high degree of probability for it. 



The improbability of the movement of the universe about the 

 earth is tripled, however, by the complete upsetting of that arrange- 

 ment which governs all the heavenly bodies whose circular motion 

 is accepted not doubtfully but with full assurance. That is, that in 

 such cases the larger the orbit the longer the time required for its 

 completion, and the smaller, the shorter. Saturn, whose course sur- 

 passes all the planets in extent, completes it in thirty years. Jupiter 

 revolves in a smaller circle in twelve years. Mars in two, the moon 

 in a month. We see clearly in the case of the Medicean stars [the 

 moons of Jupiter] that the one nearest Jupiter goes through its orbit 

 in a very short time, namely, forty-two hours, the next nearest in three 

 and a half days, the third in seven days, and the farthest removed 

 in sixteen days. This thoroughly constant rule remains unchanged 

 if we ascribe the twenty-four hour movement to the revolution of 

 the earth, but if we suppose the earth to remain unmoved, we must 

 proceed from the short period of the moon to increasingly greater 

 periods, to the two year period of Mars, the twelve year period of 

 Jupiter, the thirty year period of Saturn, and then abruptly to a 

 disproportionately larger orbit, to which must also be ascribed the 

 revolution in twenty-four hours. And these suppositions entail the 

 smallest part of the disturbance of the otherwise constant law. For 

 when one passes from the orbit of Saturn to those of the fixed stars 

 and attributes to them even greater orbits, which correspond to the 

 period of revolution of many thousands of years, one must pass from 

 this by a much more disproportionate transition to that other move- 

 ment and ascribe to them a period of revolution about the earth of 

 twenty-four hours. But if the movement of the earth is supposed, 

 the regularity of the period is accounted for in the best possible way ; 

 from the slow period of Saturn we arrive at the immovable fixed star. 



A fourth difficulty also is encountered which must be added if we 

 suppose the motion of the smaller sphere. I mean the great dis- 

 similarity in movements of these stars, some of which must revolve 



