PORTION OF THE COSMOS. THE PLANETS. 301 



the different planets belonged already to the Neo-platonic 

 Alexandrine representations of the 5th century. (Compare 

 Olympiod. Comment, in Aristot. Meteorol. cap. 7, 3 in 

 Ideler's edition of the Meteor. T. ii. p. 163 ; also T. i. pp. 

 199 and 251). 



Although the number of visible planets known to the 

 ancients amounted, according to the first limitation of 

 the term, only to five, and subsequently, when the larger 

 discs of the Sun and Moon were added, to seven; yet it was 

 already conjectured that besides these visible planets there 

 existed others, unseen because possessing only a fainter 

 degree of lustre. This opinion is pointed out by Simpli- 

 cius as an Aristotelian one : " It may be that lunar eclipses 

 are sometimes caused by such dark bodies moving round 

 the common centre as well as by the Earth." Artemidorus 

 of Ephesus, whom Strabo often refers to as a geographer, 

 believed in the existence of a countless number of such 

 dark revolving cosmical bodies. The old imaginary anti- 

 earth (avTix$<Dv) of the Pythagoreans does not belong to the 

 sphere of these conjectures. It and the Earth were supposed 

 to have a parallel concentric movement ; it was an idea 

 devised to spare the Earth, which was supposed to perform 

 around the central fire a planetary revolution in 24 hours, 

 from having also to execute a movement of rotation, and, 

 indeed, represented no doubt the opposite hemisphere, or 

 the antipodal half of our planet ( 507 ). 



If from the entire number of planetary bodies now known 

 to us, 43 primary planets and satellites, being six times as 

 many as were known to the ancients, we take the 36 which 

 have been discovered since the invention of the telescope, and 

 divide them chronologically according to the periods of 



