$ 8i] The Arrangement vf the Solar System 107 



the sun, the order of distance Mercury, Venus, Earth, 

 Mars, Jupiter, Saturn being also in accordance with the 

 rates of motion round the sun, Mercury performing its 

 revolution most rapidly (in about 88 days *), Saturn most 

 slowly (in about 30 years). On the Coppernican system 



FIG. 40. The solar system according to Coppernicus. From the 

 De Revolutionibus. 



the moon alone still revolved round the earth, being the 

 only celestial body the status of which was substantially 



* In this preliminary discussion, as in fig. 40, Coppernicus gives 

 80 days ; but in the more detailed treatment given in Book V. he 

 corrects this to 88 days. 



