$ i2 7 ] The Problem of Longitude : Comets 161 



a long ocean voyage, and some astronomical means of 

 determining the time was accordingly wanted. Galilei's 

 idea was that if the movements of Jupiter's satellites, and 

 in particular the eclipses which constantly occurred when 

 a satellite passed into Jupiter's shadow, could be predicted, 

 then a table could be prepared giving the times, according 

 to some standard place, say Rome, at which the eclipses 

 would occur, and a sailor by observing the local time 

 of an eclipse and comparing it with the time given in 

 the table could ascertain by how much his longitude 

 differed from that of Rome. It is, however, doubtful 

 whether the movements of Jupiter's satellites could at that 

 time be predicted accurately enough to make the method 

 practically useful, and in any case the negotiations came 

 to nothing. 



In 1618 three comets appeared, and Galilei was soon 

 drawn into a controversy on the subject with a Jesuit 

 of the name of Grassi. The controversy was marked by 

 the personal bitterness which was customary, and soon 

 developed so as to include larger questions of philosophy 

 and astronomy. Galilei's final contribution to it was 

 published in 1623 under the title // Saggiatore (The 

 Assayer), which dealt incidentally with the Coppernican 

 theory, though only in the indirect way which the edict 

 of 1616 rendered necessary. In a characteristic passage, 

 for example, Galilei says : 



" Since the motion attributed to the earth, which I, as a pious 

 and Catholic person, consider most false, and not to exist, 

 accommodates itself so well to explain so many and such 

 different phenomena, I shall not feel sure . . . that, false as it 

 is, it may not just as deludingly correspond with the phenomena 

 of comets " ; 



and again, in speaking of the rival systems of Coppernicus 

 and Tycho, he says : 



" Then as to the Copernican hypothesis, if by the good 

 fortune of us Catholics we had not been freed from error 

 and our blindness illuminated by the Highest Wisdom, I do 

 not believe that such grace and good fortune could have 

 been obtained by means of the reasons and observations given 

 by Tycho." 



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