BOOK II. XII. 6i-xin. 64 



limits of her distance. Mercury's stations have too 

 short a period to be perceptible. 



XIII. This is the system of the shining and 

 occultation of the planets : it is more complicated 

 from their motion and involves many remarkable 

 facts, inasmuch as they change their magnitude and 

 their colours, and both approach the North and 

 retire towards the South, and suddenly are seen 

 closer to the earth or to the sky. And although our 

 account of these matters will differ in many points 

 from that of our predecessors, we confess that credit 

 for these points also must be given to those who 

 first demonstrated the methods of investigating 

 them : only nobody must abandon the hope that the 

 generations are constantly making progress. 



All these occurrences are due to a plurahty ofduetothree 

 causes. The first is the factor of the circles which '^^^- 

 in the case of the stars the Greeks designate apsides 

 or arcs (it will be necessary to employ Greek terms). 

 Each planet has its own circle, and these are not the 

 same as those of the firmament, since the earth 

 between the two vertices, named in Greek pnles, is 

 the centre of the sky, and also of the zodiac, which 

 is situated on a slant between the poles. [All these 

 facts are always estabhshed beyond doubt by the 

 method of compasses."] Therefore the special arc 

 of each is drawn from a different centre, and conse- 

 quently they have different orbits and dissimilar 

 motions, because the inner arcs must necessarily be 

 shorter. 



It follows that the points of the arcs highest above 

 the centre of the earth are : in the case of Saturn 

 in Scorpio, in that of Jupiter in Virgo, of Mars in 

 Leo, of the sun in the Twins, of Venus in the Archer, 



211 



