$$ 57 6o] The Bagdad School: Albc.tcgnins 79 



the defects in the Greek astronomical tables, and new tables 

 were from time to time issued, based on much the same 

 principles as those in the Almagest, but with changes in 

 such numerical data as the relative sizes of the various 

 circles, the positions of the apogees, and the inclinations 

 ,of the planes, etc. 



To Tabit ben Korra, mentioned above as the translator of 

 the Almagest, belongs the doubtful honour of the discovery 

 of a supposed variation in the amount of the precession 

 (chapter n., 42, 50). To account for this he devised a 

 complicated mechanism which produced a certain alteration 

 in the position of the ecliptic, thus introducing a purely 

 imaginary complication, known as the trepidation, which 

 confused and obscured most of the astronomical tables 

 issued during the next five or six centuries. 



59. A far greater astronomer than any of those mentioned 

 in the preceding articles was the Arab prince called 

 from his birthplace Al Battani, and better known by the 

 Latinised name Albategnius, who carried on observations 

 from 878 to 918 and died in 929. He tested many of 

 Ptolemy's results by fresh observations, and obtained 

 more accurate values of the obliquity of the ecliptic 

 (chapter i., u) and of precession. He wrote also a 

 treatise on astronomy which contained improved tables 

 of the sun and moon, and included his most notable dis- 

 covery namely, that the direction of the point in the 

 sun's orbit at which it is farthest from the earth (the 

 apogee), or, in other words, the direction of the centre of 

 the eccentric representing the sun's motion (chapter n., 

 39), was not the same as that given in the Almagest ; 

 from which change, too great to be attributed to mere 

 errors of observation or calculation, it might fairly be 

 inferred that the apogee was slowly moving, a result which, 

 however, he did not explicitly state. Albategnius was also 

 a good mathematician, and the author of some notable 

 improvements in methods of calculation.* 



60. The last of the Bagdad astronomers was Abul Wafa 







* He introduced into trigonometry the use of sines, and made also 

 some little use of tangents, without apparently realising their im- 

 portance : he also used some new formulas for the solution of 

 spherical triangles. 



