240 THE GREEK ASTRONOMY. 



of the Greeks ; and also because they illustrate 

 the permanence or variability of important ele- 

 ments, such as the obliquity of the ecliptic and the 

 inclination of the moon's orbit. 



We must, however, notice one or two peculiar 

 Arabian doctrines. The most important of these 

 is the discovery of the Motion of the Sun's Apogee 

 by Albategnius. He found the Apogee to be in 

 longitude 82 degrees; Ptolemy had placed it in 

 longitude 65 degrees. The difference of 17 degrees 

 was beyond all limit of probable errour of calcu- 

 lation, though the process is not capable of great 

 precision ; and the inference of the Motion of the 

 Apogee was so obvious, that we cannot agree with 

 Delambre, in doubting or extenuating the claim 

 of Albategnius to this discovery, on the ground 

 of his not having expressly stated it. 



In detecting this motion, the Arabian astrono- 

 mers reasoned rightly from facts well observed ; 

 they were not always so fortunate. Arzachel, in 

 the llth century, found the apogee of the sun to 

 be less advanced than Albategnius had found it, 

 by some degrees; he inferred that it had receded in 

 the intermediate time ; but we now know, from an 

 acquaintance with its real rate of moving, that the 

 true inference would have been, that Albategnius, 

 whose method was less trustworthy than that of 

 Arzachel, had made an errour to the amount of the 

 difference thus arising. A curious, but utterly false 

 hypothesis was founded on observations thus erro- 



