224 A SHORT HISTORY OF SCIENCE 



showed rotation of that body, the satellites of Jupiter and par- 

 ticularly the phases of Venus, analogous to those shown by the 

 moon, obviously harmonized with the Copernican theory. This 

 implied at least that the planets shone by reflected sunlight, and 

 it had indeed been insisted against that theory that Venus and 

 Mercury under it must show phases till then undiscovered. 



In 1632 Galileo published his celebrated Dialogue on the Two 

 Chief Systems of the World, the Ptolemaic and the Copernican, 

 a work comparable in magnitude and importance with Copernicus' 

 Revolutions. In the curious preface he says : 



* Judicious reader, there was published some years since in Rome a 

 salutiferous Edict, that, for the obviating of the dangerous Scandals 

 of the present Age, imposed a reasonable Silence upon the Pythag- 

 orean Opinion of the Mobility of the Earth. There want not such as 

 unadvisedly affirm, that the Decree was not the production of a sober 

 Scrutiny, but of an informed passion ; and one may hear some mutter 

 that Consultors altogether ignorant of Astronomical observations 

 ought not to clipp the wings of speculative wits with rash prohibitions. 

 My zeale cannot keep silence when I hear these inconsiderate com- 

 plaints. I thought fit, as being thoroughly acquainted with that 

 prudent Determination, to appear openly upon the Theatre of the 

 World as a Witness of the naked Truth. ... I hope that by these 

 considerations the world will know that if other Nations have Navi- 

 gated more than we, we have not studied less than they; and that 

 our returning to assert the Earth's stability, and to take the contrary 

 only for a Mathematical Capriccio, proceeds not from inadvertency 

 of what others have thought thereof, but (had one no other induce- 

 ments), from these reasons that Piety, Religion, the Knowledge of the 

 Divine Omnipotency, and a consciousness of the incapacity of man's 

 understanding dictate unto us.' 



In the first of the four conversations into which the work is 

 divided, the Aristotelian theory of the peculiar character of the 

 heavenly bodies is subjected to destructive criticism, with em- 

 phasis on such phenomena as the appearance of new stars, of 

 comets and of sun spots, the irregularities of the moon's surface, 

 the phases of Venus, the satellites of Jupiter, etc. 



