SEQUEL TO THE EPOCH OF H1PPARCHUS. 241 



neously appreciated; namely, the Trepidation of 

 the fixed stars. Arzachel conceived that a uniform 

 Precession of the equinoctial points would not ac- 

 count for the apparent changes of position of the 

 stars, and that for this purpose, it was necessary 

 to conceive two circles of about 8 degrees radius 

 described round the equinoctial points of the im- 

 moveable sphere, and to suppose the first points of 

 Aries and Libra to describe the circumferences of 

 these circles in about 800 years. This would pro- 

 duce, at one time a progression, and at another 

 a regression, of the apparent equinoxes, and would 

 moreover change the latitudes of the stars. Such 

 a motion is entirely visionary ; but the doctrine 

 made a sect among astronomers, and was adopted 

 in the first edition of the Alphonsine Tables, though 

 afterwards rejected. 



An important exception to the general unpro- 

 gressive character of Arabian science has been 

 pointed out recently by M. Sedillot 11 . It appears 

 that Mohammed- Aboul Wefa-al-Bouzdjani, an Ara- 

 bian astronomer of the tenth century, who resided 

 at Cairo, and observed at Bagdad in 975, dis- 

 covered a third inequality of the moon, in addition 

 to the two expounded by Ptolemy, the Equation 

 of the Center, and the Evection. This third inequa- 

 lity, the Variation, is usually supposed to have 

 been discovered by Tycho Brahe, six centuries 



41 Sedillot, Nouvelles Rech. sur 1'Hist. de 1'Astron. chez les 

 Arabes. Nouvean Journal Asialif/ue. 1836. 



VOL. I. R 



