100 THE LIFE AND LEGEND OF MICHAEL SCOT 



scholar of the celebrated Ibn Tofail, the author of 

 the curious Sufic romance called Hay Ihn Yokhdan. 



In the preface to his book on the Sphere Alpe- 

 trongi begs to be excused if he has ventured to 

 differ from the tradition of the ancients in his 

 theory of the heavenly movements, and especially 

 from Ptolemy the great master of this science. 

 His apology reminds us that it may be well to 

 examine more exactly than we have yet done the 

 various advances which had been made up to this 

 time by the Arabian astronomy. 



As early as the ninth century the mathe- 

 maticians of that nation had simplified the problems 

 of the circle by discovering the way of measure- 

 ment by sine and tangent instead of by the chord. 

 This improvement is ascribed to Albategni who lived 

 between the years 877 and 929. Calculation was 

 soon made still easier by the invention of algebra. 

 The year 820 is given as the age of Mohammed ben 

 Moussa, surnamed Al Khowaresmi, who had the 

 honour of this important discovery. From the 

 surname of this mathematician the Latins after- 

 wards formed by corruption their common noun 

 Algorisma or Algorithmus, from which our word 

 arithmetic is derived. 



These improved methods of calculation were 

 soon applied to astronomy. Al Mamun, whose reign 

 commenced in the year 813, summoned an assembly 

 of scholars learned in that science. They met in the 

 great Babylonian plain, having chosen that place as 

 suitable for their observations, and measured the de- 

 clination of the ecliptic, which they determined to be 

 23° 33". About the same time the secular motion of 

 the heavens began to attract attention. Albategni 



