BOOK XVIII. Lvn. 215-Lviii. 218 



lonia, Asia and the islands belonging to it ; when 

 the Morea, Achaia and the hxnds lying to the west 

 of it ; and the term ' Chaldaeans ' will indicate 

 Assyria and Babylonia. That the names of Africa 

 and the provinces of Spain and Gaul are not men- 

 tioned will cause no surprise, because none of those 

 who have published accounts of the risings of the 

 constellations have made observations in respect of 

 those countries. Still it will not involve a diffieult 

 calcuhition to ascertain them in those countries as 

 well, by means of the explanation of parallels which 

 we have set out in Book Six, which indicates the vi. 212 u. 

 astronomical relationship not onlv of nations but of 

 individual cities as well. Thercfore by taking the 

 circular parallel belonging to the countries we have 

 specified and applying it to those that the par- 

 ticular student is seeking, the risings of the constel- 

 lations will be the same throughout the parts of all 

 the parallels where shadows are of equal length. It 

 is also necessary to point out that the seasons them- 

 selves have their own periods every four years, and 

 that they too return without great variation under 

 the system of th(; sun, but that they are also 

 lengthened every eight years at the hundredth 

 revolution of the moon. 



LVIII. The whole system however is based on nuingand 

 three lines of observation — the rising and the setting ^consieiia- 

 of the constellations and the periods of the seasons """*• 

 themselves : there are two modes of observing the 

 risings and settings, as the stars are either hidden by 

 the arrival of the sun and cease to be visible, or they 

 present themselves to the view on the sun's departure 

 (so that custom would have done better to designate 

 the latter as the stars' ' emergence ' rather than 



327 



