198 



A SHORT HISTORY OF SCIENCE 



The new explanation obviously accounted also for the variations 

 in the brightness of these planets. 



' It is certain, ' he says, ' that Saturn, Jupiter and Mars are always 

 nearest the earth when they rise in the evening, that is when they are 

 in opposition to the sun, as the earth is situated between them and 



THE COPERNICAN SYSTEM 



the sun. On the contrary, Mars and Jupiter are farthest from the 

 earth when they set in the evening, the sun lying between them and 

 us. This proves sufficiently that the sun is the centre of their orbits, 

 as of those of Venus and Mercury. Since thus all planets move 

 about one centre it is necessary that the space which remains between 

 the circles of Venus and Mars, contain the earth and its accompanying 

 moon.' 



He is, therefore, not afraid to maintain that the earth with the 

 moon encircling it, traverses a great circle in its annual motion 

 among the planets about the sun. The universe, however, is so 

 vast, that the distances of the planets from the sun are insignificant 



