A HISTORY OF SCIENCE 



same way if, instead of disturbing so large a part of 

 the universe, you let the earth revolve about itself. 

 The difficulty is, however, doubled, inasmuch as a 

 second very important problem presents itself. If, 

 namely, that powerful motion is ascribed to the heav- 

 ens, it is absolutely necessary to regard it as opposed 

 to the individual motion of all the planets, every one 

 of which indubitably has its own very leisurely and 

 moderate movement from west to east. If, on the 

 other hand, you let the earth move about itself, this 

 opposition of motion disappears. 



"The improbability is tripled by the complete over- 

 throw of that order which rules all the heavenly bodies 

 in which the revolving motion is definitely established. 

 The greater the sphere is in such a case, so much longer 

 is the time required for its revolution ; the smaller the 

 sphere the shorter the time. Saturn, whose orbit sur- 

 passes those of all the planets in size, traverses it in 

 thirty years. Jupiter 4 completes its smaller course in 

 twelve years, Mars in two ; the moon performs its much 

 smaller revolution within a month. Just as clearly in 

 the Medicean stars, we see that the one nearest Jupiter 

 completes its revolution in a very short time about 

 forty-two hours ; the next in about three and one-half 

 days, the third in seven, and the most distant one in 

 sixteen days. This rule, which is followed through- 

 out, will still remain if we ascribe the twenty-four- 

 hourly motion to a rotation of the earth. If, however, 

 the earth is left motionless, we must go first from the 

 very short rule of the moon to ever greater ones -to the 

 two-yearly rule of Mars, from that to the twelve -yearly 

 one of Jupiter, from here to the thirty-yearly one of 



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