THE ASTRONOMICAL WRITINGS OF SCOT 101 



corrected the observations of Ptolemy here, and 

 showed that the retrograde movement amounted to 

 one degree, not in a century as the Greek philosopher 

 lad said, but in a shorter period which is variously 

 itated as sixty-six or seventy years. Alfargan re- 

 lated this calculation, and amended that relating 

 ,o the declination of the ecliptic, which he computed 

 it 23 35". 



This was the progress and these the data which 



ed the Moorish astronomers to abandon the earlier 



and simpler theories of the sphere as inconsistent 



with ascertained facts. They were aware of 



motions among the heavenly bodies not to be 



explained by the mere supposition that round the 



arth as a centre moved the concentric spheres 



on the axes of their poles. It is true that 



sven Ptolemy himself had felt something of this 



difficulty and had endeavoured to meet it by a 



;heory of eccentrics and epicycles. As knowledge 



increased, however, this primitive explanation was 



Lt to be cumbrous and unsatisfactory. Aboasar 1 



,nd Azarchel gained fame by boldly striking out in 



new paths, and later Moorish astronomers eagerly 



bllowed the lead thus given them, each adding 



some modification of his own. 



Thus then we return to the preface of Alpetrongi 

 Drepared to understand his position when he 

 leclares himself obliged to depart from previous 

 ;raditions. He proceeds to avow himself a scholar 

 >f Azarchel, but when we examine his work we find 

 .hat the theory he proposes differs considerably 

 ;ven from that taught by his immediate master. 



1 For the unfavourable judgment of Mirandola on this astronomer, 

 ee infra, p. 143. 



