102 THE LIFE AND LEGEND OF MICHAEL SCOT 



It was one which, through the labours of Michael 

 Scot, as translator of Alpetrongi, exercised no small 

 influence on the study of astronomy among the 

 Latins, and we may well spend a moment in con- 

 sidering the chief features which it presents. 



One of the most important problems which 

 called for solution at the hands of the Moorish 

 astronomers was that of the recession of the 

 heavenly bodies, by which, when observed at 

 sufficient intervals of time, they were seen to fall 

 short of the positions they might have been 

 expected to reach. This recession, as we have 

 remarked already, had been very accurately studied, 

 and computed as exactly as the methods of the 

 time allowed ; but a reason for so remarkable a 

 phenomenon was yet to seek. Alpetrongi boldly 

 declared that the eastward motion was apparent 

 only and not real. He explained that the source 

 of power lay in the primum mobile or ninth sphere ; 

 that lying outside the sphere of the fixed stars. 

 From hence the force producing circular motion 

 was derived to the eighth, and so to the inferior 

 spheres ; each handing on a part of the impulse 

 to that which lay beneath it. In the course of 

 transmission, however, the prime force became 

 gradually exhausted. Thus, said Alpetrongi, it 

 happens that each sphere moves rather more 

 slowly than the one above it, and so the apparent 

 recession is accounted for in a way which shows it 

 to be relative only and not absolute. 



Another matter which exercised the minds of 

 those who studied the heavens was the difference 

 of elevation which the heavenly bodies showed 

 according to the seasons of summer and winter. 



